The real juice of modded minecraft is in the modpacks - curated sets of mods that were configured to work well with each other, frequently with some custom recipes added by the pack developer, and sometimes some kind of a quest line to guide you through the pack and provide a more structured experience. There are many different types of modpacks - kitchen sinks (large collections of mods, frequently without a lot of balance tweaks or changes, for a more sandbox experience), questing packs (with the aforementioned quest books to guide you through the mods), vanilla+ packs that intend to expand on the vanilla minecraft experience and not change the gameplay loop significantly, packs focused exclusively on magic or technology mods (or both), expert packs (questing packs with heavily reworked recipes, where you need to build elaborate machines and automate stuff Factorio-style)...

I'm not up to date with the modpack scene, so can't really make you a definitive list - back on reddit (sigh) there is a r/feedthebeast community that specializes in modded play.

That said:

• FTB Academy seems to be a pack specifically meant to teach the basics of modded play.
• Project Ozone 3 comes up quite often as a pack with a good quest book that guides you through everything.
• Cottage Witch is what I'm currently starting, it's (so far) a chill magic vanilla+ pack. New creatures, new plants, some new mechanics, tons of new decorations for building.
• Peace of Mind is an older pack made specifically for playing on Peaceful, if mobs are stressing you out. It's got a good questbook too.
• and if you want to jump straight into the deep end... Enigmatica 2 (or 6) Expert, Gregtech New Horizons. Expert packs in which you need to automate everything to progress. Gregtech in particular is infamous for its complexity, difficulty, and length, but if you enjoy solving hard problems it might be for you.

You'll also need a launcher to install these packs - FTB have their own if you want FTB Academy, otherwise there are some options such as Curseforge (do not recommend, eats resources just by existing), Prism (seems to come up a lot as a recommendation), or GDLauncher (what I'm using).

Yeah what the other guy said, modpacks and give FTB academy a start. Generally the mods add a shitload of new content (like lots more ores). Better automation and electricity is, imo, the best stuff added, and there's tons of that. I find the magic and adventure mods don't quite work as well. My biggest tip for modded mc is: Spread out! Make big ass bases and rooms, you'll love the space.

After that it's your call what's next. A kitchen sink pack is one that sorta rams in a ton of mods with no theme and it's fun! FTB infinity was a lot of fun, or FTB Ultimate re whatever too.

There's StoneBlock which is the opposite of Skyblock which was a different style

Create: Above and Beyond is my favorite. It is hard though and requires that you understand the Create mod.

By the way you'll find that Create is the best mod. It's really fucking well done and no other mod really comes close in quality. Gears and belts!

Anyways here's some old creations of mine:

Big bridge that runs off trees and wheat https://imgur.com/gallery/HeIl4vk

Storage room (before I got the ME computer block) https://imgur.com/gallery/U0qhf

Create wheat farm in the snow https://imgur.com/gallery/x71winR

This last pack I played was all about big multi block structures to process ore, so it became a sprawling base https://imgur.com/gallery/873rZT5

There’s a few different styles of experiences:

The Aether

Adds a dimension in the sky with its own progression of ores, and a system of a progression of dungeons. Lots of new enemies. It has a kinda similar progression to playing vanilla survival minecraft, but it’s harder and the things you have to worry about are very different.

It’s one of the most polished mods out there and is intended for a standalone experience.

Mine & Slash this is a big modpack intended to change the game into a more combat oriented and fantasy themed game.

There are some that are designed to make the progression be a system of automating resource production, similar to games like Factorio or Satisfactory. Create is an example.

Ones like Blightfall are a complete curated experience with a story, a custom map, and a modpack.