I would personally recommend you start over at level 1, and discard any notions of how things might work that you carry over from other systems (even besides 5e).
In general for TTRPGs in the larger sphere, every system is its own thing, and looking at rules with fresh eyes and no preconceived notions of how it might work tends to help interpret and understand them better without tripping yourself up on what you thought they might be.
The rules are legally free and you can check them out in places like the Second Edition section of Archives of Nethys and PF2eTools (if that name sounds familiar and you have concerns, worry not! They have contact with Paizo to keep them in the clear and avoid content that doesn't fall under the free use rules, and were even present at PaizoCon 2023!).
If you want an excellent tutorial to get started, the Beginner Box is great at teaching as you go. Hope you enjoy the system!
Regarding number one - If you're coming back from a long time away and haven't done any quests at all, then I recommend doing quests. They unlock important content, and there's a few that are necessary to be able to do Eidolons properly.
As for the remainder of the questions, if you care about meta (or at least some form of guide on making weapons work) and the performance of weapons/frames then I recommend you check out the tier lists and build guides on Overframe. Not all super upvoted builds are up to date or great, so I recommend you look around on each piece of equipment, read the comments on the builds, and scrutinize for yourself on what you think might work and be good.