Welcome back, Tenno. So, I'm sure people have made step by step guides, but the reality is that you're going to want to dip your toe into everything you can that interests you. Even some stuff that doesn't interest you.
The most bog standard, but not incorrect, "new player mid term goal" is "Complete the Star Chart". It's something you'll hear people say all the time, but it's for a good reason. They made it such that you HAVE to do main story quests to progress through the planets now, and you'll by nature learn what every new mission type is, start farming some new gear, dip your toe into every one of the bounty zones. By the time you finish it you'll have a good overview of what the game has to offer, as well as unlocking almost all the alternate game modes, including Arbitrations and Steel Path.
I'd recommend building everything you can along the way, every weapon and every warframe, companions, etc. Just getting a lot of that grunt work done so you can cash in on the mastery rank is nice. MR doesn't equate to skill or proficiency at the game, but until MR16 some content will straight up be locked out which isn't a system I agree with but it's the way it works.
To address a few of the things you asked about specifically; Railjack is mostly a content island, you need it for some quests but it's its own thing. Archwing missions are kind of dead old content, but having access to the Itzal and Amesha archwings themselves are nice because they can be used in other content.
Void Keys don't exist anymore, and have been replaced with Void Relics, which is how you get prime parts to sell for platinum or build to get access to weapons and warframes. It's not my favorite system because it reduces all primes to the same one kind of farm, but it does keep it a lively and healthy community so farming those missions is never hard to do.
Tenno Powers aka Focus Schools will be something you largely do passively these days. They're incredibly powerful and useful now post focus 3.0, which while I have my problems with it I think it was overall a healthy change. Certain new enemy types will just give you sizable chunks of Focus when you kill them, so you no longer have to do eidolon hunts to make any real progress. Zenurik used to be the old golden standard because it gave you access to energy regen and that's not bad, but all of them are useful now. Madurai is my standard equip. You can get a good amount of focus just by playing the Zariman missions, but you get a ton from Duviri as well.
PvP is mostly dead, and Rivens are kind of a mess. Rivens can help certain specific weapons, in certain specific contexts, enable builds that wouldn't otherwise be possible, but in terms of rivens for just raw power increase at what the weapon already does they're kind of irrelevant and not something you need to worry about.