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.

1. Always do quests first. You unlock more options to use in your general gameplay through them.

2. Incarnon weapons are meta right now. They get unlocked either through randomised Steel Path mode in Duviri, called Circuit, or through Zariman, which should be unlocked through one of the last available quests. Alternatively, you can opt to get yourself an adversary Kuva Lich (Grineer) or Sister of Parvos (Corpus), who have the second best weapons in the game. They are also unlocked through quests but rather early ones.

3. Depends on what you want the frame to do. Nuke? Saryn. Survivability? Revenant. Speed? Titania. Ability damage? Protea. Support? Wisp. Defending? Frost. Stealth? Wukong (no really). Assassination? Ivara. Hotel? Trivago.

4. See 2 and 3 but as long as it’s well modded and used properly, nearly anything can handle Steel Path.

1. Do the quest first. You might want to farm a basic amp if you haven’t done Chains and the Harrow and The Sacrifice yet, since those are harder with a mote amp.
   2.The META weapons vary depending on activity, but Incarnon weapons are currently the strongest.
2. Very much depends on role and mission. Each frame excels in its own way. But right Now Revenant is best for just not dying no matter what is thrown at you. Wisp is the best general support frames for stationary missions. Saryn is still the best high enemy density endurance nuker and that’s unlikely to change. Trinity is still the best source of energy for the whole squad.

Quests first, even the side quests unlock content on occasion. Personal opinion here is to leave eidolons last, better to use easier to farm arcanes from Zariman/mirror defense/lua for now

Incarnon weapons, but those are a ways off given you need steel path. Zariman after quests also has some weapons.

All but the absolute trash frames, weapons will carry the game on their own. Depends on playstyle. Me personally will only use frames with 90% DR or projectile reflection since i dont like dying all the time.

Up until level 180-200 meta weapons with as much upfront damage as you can muster, after that viral slash.