First of all use for 1st character a farming character like Axton or Maya aka commando or siren. i can’t answer number 1 cause Im playing my axton again which is non elemental and i’ve forgoten all about DOT cause my gaige is the one using elemental weapons.
2 No you should farm only when your LVL 50 endgame of TVHM, LVL 72 endgame of UVHM, OP5 cause enemies will be hard in OP6 - OP8, OP8 cause its the final lvl you can get in this game. To unlock OP levels must beat digistruct peak to unlock OP1 then beat it again to gain next level so on until OP 3 or OP 4 wherein the final boss will unlock now you must defeat it to progress OP 5 to OP 8
3 golden keys are pretty useless in this game unless you don’t have time for farming
Tip wait till lvl 50 to do sidequests in TVHM lvl 72 in UVHM reset UVHM in OP 5,reset again UVHM in OP8. Reason for this is so you can get gear that is in missions only