I'd recommend Beyond All Reason. It's open-source and utilizes a realistic scifi theme. You essentially manage metal and energy to produce more units, to fight against other player's units. It's very easy to learn, and looks nice graphically. The only caveat I can think of is that it is not on Steam. Game Link: https://www.beyondallreason.info/

StarCraft II. It's on Battle.Net though so you'll have to download it through Lutris.

The first campaign is free and there's a coop mode for casual players. Entirely free except for cosmetics, some coop commanders, and the other campaign episodes. It's easy to pick up IMO but VERY HARD to master. Graphics still hold up better than a lot of games despite being 13 years old. Despite its age, I believe it's the peak of the RTS genre. There's are a lot of community-made mods/maps that you can play for free. You can even play a remake of the WarCraft III campaign there.

Other RTS games that I like:

• Age of Empires II (Steam). Graphics look decent enough for the remastered version on Steam. Complex economy system and slow rock-paper-scissors combat. Very slow-paced.
• StarCraft Broodwar (Battle.Net). Remastered graphics also look decent enough. Unit pathing might still have annoying quirks like the original. Slightly slower-paced than StarCraft II.
• Northgard (Steam). Beautiful game with a unique event-type economy system. Slower-paced than StarCraft II with some colony-building elements.

Not really an RTS game but I've been putting so many hours into RimWorld (Steam, native) this year. At its core, it's a colony-building game but it does have some RTS elements to it (economy, combat, and unit management). Lots of mods available.

command and conquer remastered.

Why not start with the best, greatest, and one of the progenitors of the entire genre?

I recently checked out BAR and liked it. I don't like micro in RTS games, because I always think "a computer can do this better than I could", so it's nice that they've got good unit automations available.

Starcraft 2 is the solid bet, the campaign teaches you a lot.

But Supreme commander forged alliance for just a crazy amount of fun, almost 20 year old game and still has the game play to keep you landed, and they have YouTube games online.

Your Linux distro probably packages 0ad, which is free and open source.

https://play0ad.com/

It also probably also packages spring, an RTS engine -- originally TA Spring, based on Total Annihilation, and it looks like it's also on Steam. Can play a couple of RTSes that use it, like Zero-K, which is similar to Total Annihilation.

https://en.wikipedia.org/wiki/Zero-K

Age of Empires 2.

How has nobody recommended Supreme Commander (Forged Alliance)? It's the inspiration for other games in this thread like Planetary Annihilation and Beyond All Reason. It's so good that when the official servers shut down the community banded together and formed their own server and dev team to continue regular balancing and updates. It's on steam and works on Linux with minimal tinkering. You can either play the campaign on the Steam version or head straight over to Forged Alliance Forever, the community-hosted server, to access the campaign (with added multiplayer support!), all the new patches and mods, and play some multiplayer games too.

I was and still am a big fan of the original Warhammer 40k: dawn of war.

I'm gonna recommend Northgard. It's a tile-based RTS with modern sprite graphics, as opposed to some games like Starcraft 2 or 0 AD (a free/libre game) that use 3d graphics. So, in some senses, it looks more indy or dated than a lot of modern offerings.

But it has really good gameplay, good base clans, good DLC, and is consistently updated. This can mean balancing/clan changes, but I must say that even when there was significant balancing action on my favorite clans, I come to accept and like the changes. Major DLC releases also come with content updates for the main game, and it really feels like Northgard is a labor of love.

It has a story mode that starts out easy-peasy and slowly ramps things up as you get better. It has multiplayer vs friends, random onliners, and/or bots of various difficulties. It has challenge modes per-clan that unlock new cosmetics.

Gameplay:

There's a lot of complexity here, but the campaign does a really good job of introducing it at a rate that's palatable.

This is a brief overview, mainly to show that, though Northgard may have simple graphics, it's really an in-depth and balanced RTS.

• Map tiles: you start with one, and acquire more.
• Seasons: Winter is a bitch. Always be extra-prepared for a bad winter.
• Random events: stuff to join in on or avoid. Rats, undead or ghost attacks, kraken, ceremonial bonfire, mine for the dwarves, exploding volcanoes that drop rock baddies that you can mine for stone
• Neutral factions: earn their favor to avoid their ire or to become their allies. Giants, kobolds, a wyvern, dwarves, myrkalfar - all have different potential benefits if you earn them.
• knowledges: learn and improve skills, gain clan specials, reduce costs - this is basically the tech tree.
• win conditions: domination, fame, economic, and even clan-specific or map-specific win conditions

Resources:

• happiness - earned through various means - nonphysical, not storeable - when positive, people work hard and new colonists come to the colony. When negative, people slack off and no new colonists arrive. Each colonist takes up happiness, except units that generate happiness, like bartenders.
• lore - earned by loremasters and various means - nonphysical, storeable - buy knowledges
• food - earned by food-producing units like farmers, fishers, hunters, healers, and idle colonists - storeable, infinite supply - all colonists need food to survive. Also used for colonizing new tiles and holding feasts (boost production and provide fame)
• wood - earned by woodcutters - storeable, infinite supply - build and repair structures. Keep people warm in winter.
• krowns - earned by sailors, trading posts, markets - storeable, infinite supply - money. Used in all kinds of areas. Used in training military units.
• stone - earned by miners - storeable, limited supply - building upgrades
• iron - earned by miners - storeable, limited supply - unit upgrades, hero spawn
• military experience: level up your military along an attack, defense, or strategic path
• fame: level up clan skills
• economic influence: it's complicated

Regardless of all of that complexity, Northgard manages to sum it all up into engaging, understandable gameplay, with clear stats on who's made the most progress on win conditions, etc, and a straightforward and manageable learning curve.

You can undergo cascades when things don't go right - a winter freezing without wood leaves everyone in poor health, often unhappy, performing at a fraction of their usual production - thus nobody's needs are met and everybody stays unhappy and underperforming. A 'simple' fix is to make everyone into regular villagers again, then farmers, woodcutters, healers, and other roles as you can afford them.

Also - Linux native.

Since some other good recommendations were already made, I'm gonna put something a bit different here: Tooth and Tail. It's not a classic RTS, and you can't really micro your units like in other RTS games, but it's fun and fast paced. Games only last 15-ish minutes at most and the gameplay is relatively simple. It's 2D so it probably also runs on anything from smart toaster to supercomputer.

I’m not 100% sure if this meets any of your requirements but I always enjoyed homeworld can’t remember if I’ve ever run it on Linux since switching tho.

As to the other requirements, I don’t think I found it that easy to get into as it took a while to get into the whole 3D map thing, and the graphics may have aged

Chiming in with Starcraft 2 (and 1). You can run it through Lutris like I set up for my friend or Steam which I use myself. The game runs great, it's highly polished, and still has a big community and pro scene. Somebody here complained about getting stomped in competitive, but both games use an MMR system so after some placement matches you'll get paired with roughly even opponents who are just as bad as you are :) There's also a (tiny but slowly growing) Starcraft community here on Lemmy and many on Discord. There are also more casual team games and co-op commander games as well as many popular custom game types/maps. One of my favorites is Battle Poker where your cards are units and the winner is determined by an automatic battle between player's units who don't fold.

I always liked planetary annihilation. It is really easy, looks decent, and has a native Linux version. That it's placed in a planet system makes it different to many other RTS.

Spellforce 3, just picked it up today and it's pretty easy to learn for the most part. It's very much like warcraft 3 in gameplay style but looks great graphically. It's also on sale right now on steam.

StarCraft 2, it's a classic and relatively easy to pick up. The only unfortunate thing about it it's that is on battle.net, great have other than that

World in Conflict

Warno

I know many don’t like it, but Supreme Commander 2 is very fun 1V1. It’s a good mix between build engine and classical RTS.

Just don’t play the campaign, its a disaster.

Factorio