Is there a new player guide on what to do / focus on?
While not technically a new player, I might as well be. I played back when Loki was my starter frame, and I don't remember the planets being locked on the star chart. I have started over on a fresh account to try to learn the game, but holy cow there's so much going on I am completely overwhelmed.
Do I need to really learn the following systems? Can I ignore any of these until later on, or do I need to focus on some now?
Railjack
Archwing
Void Keys
Fishing
Mining
Building my own weapons/pets/robot companions
Levelling up my Tenno powers
Conclave PVP
Riven mods
Trying to figure all this out while getting mastery ranks to unlock more trades and new weapons is a lot of mental overhead. Also, will there be stuff in the lategame that is more challenging and requires me to actually upgrade stuff? It's pretty easy so far.
Not that I'm aware, but then I also don't like too much watching video guides unless they are on a very very specific topic and they are also very very concise.
I play since closed beta but I'm on and off ...even for loooong periods of time, like as 3 years in the case of my last hiatus, and for anything I don't know I just search the wiki: https://warframe.fandom.com
As for your questions:
Railjack
Don't mind about it for now, you can play also without by joining other crews. It used to need belonging to a clan to have one, don't know if it is still the same (my clan luckily never kicked me for inactivity so I don't know)
Archwing
Most of it got integrated into RJ, while AW Guns can be used temporarily also in open world missions, or mounted on .... another thing
Void Keys
They exists no more and they have been converted in relics. Each relics has a specific loot pool disregarding the mission they are played with. In practice DE moved the prime loot pool from specific mission types and planets to specific atrociously named relics
Fishing
It is an activity you can do in open world maps, it can help you level up the relative faction and by doing that you usually can also improve its possibilities too; mats are used for crafting gear
Mining
same as fishing
Building my own weapons/pets/robot companions
They rely on levelling up the open world factions, needs fishing/mining ingredients and also conservation ingredients too sometimes. That is another activity like hunting but non-lethal and finalized to the safe capture of endangered species.
Levelling up my Tenno powers
Focus? Faster farm afaik now is doing zariman missions, but you need to finish the quests in order to open that content
Conclave PVP
what?
Riven mods
I never use them or feel the need for them, I don'0t think they are needed in any content, but it might be good way to get some plat by trading them
and I'll add Duviri if you picked that way to start:
it is a kind of rogue-lite game mode, quite simple and handy for new players.
As for in general I'd focus on unlocking the star chart (and the relative junctions!) and playing the quests. Have fun tenno.