For modding, I typically just go default damage mod, multishot, 60/60s for cold, toxin, and heat (to have viral + heat) and that's usually about all you can get away with before being out of points without a catalyst/forma. If you want an easy build without having to do a ton of testing, you can always go to overframe.gg and see what people are suggesting. Most builds people post will be using fully-ranked primed mods and a bunch of forma but you can see what works and pare it down to something you can use.

Yeah, Railjack is really weird to level because weapon and hull strength (mostly) come from enemy drops in missions (there are also a few clan unlocks for the alt-fire IIRC), and actually modding the railjack itself does relatively little. This means that if you can get taxi'd by someone on a later railjack mission you'll unlock a bunch of tier 3 parts and be set, but if you do it entirely on your own it's a fairly miserable and prolonged grind. AI crew are great once you get them because they can just stay on the ship and repair it, but they're locked behind some intrinsics IIRC so you have to suffer for a while. Generally I'd say corpus missions are easier than grineer, but I'm not sure if you start with them unlocked.