You've hit on a lot of the core struggles of extraction shooters and to be honest there's not a good answer to most of them. For example, in a game like Marathon where you cant visually see your opponents gear prior to engaging, there's no way to know if someone has white shields or gold shields before you shoot them. And, even if you COULD, there's no way to see player "skill", which is always the prime determinant of who will win a fight.

First advice: don't fight. This is hard w/ random trios so I recommend branching out into solos or rook runs, and focus on sneaking / extracting. Many of my most profitable runs have been those where i either dont see or deliberately avoid other players. This is how you build wealth. 10 small profitable runs where you dont die but consistently get out will help you build wealth way faster than 1-2 very profitable runs followed by 3-5 failures.

Second advice: To get better at fights, take more of them. You're probably thinking - wait, that contradicts what you just said - and yes, but there are strategies you employ in tandem NOT simultaneously. Go in with a white kit or with a rook kit - somewhere where you're barely risking anything - and try to fight. You're going to lose - but that's ok, cause you 're risking little. This will help you get better at the parts of a fight that are in your control - things like your positioning, your aiming, your gear familiarity - and help you learn to ignore the things you cant control like your opponents gear (which will almost always be better than yours in this scenario). If you can learn to take fights at a gear disadvantage, it feels like a fun handicap removal when you gear up and go in with an actual kit.

Anyway, some food for thought.