DrDamn iLevel is def the best indicator, although for weapons the only stat that really matters is damage. Any gear you get now will almost certainly be replaced instantly by basic level 50 vendor gear when you get to HW so don’t worry about it too much unless there’s something you want for cosmetics.
Most of the optional dungeons/trials that you unlock via quests aren’t part of the standard levelling roulette so you only need to unlock them if you want to run the 50/60/70/80 roulette, you won’t queue into them otherwise.
Relic weapons are practically a whole game in themselves. The quests to get each of them are very long and some of them needs a ton of expensive/rare materials. Aside from the most recently added relic weapon (and depending on where we are in the release cycle, sometimes not even that), they’re only used for glamour - the stats on them are totally outdated. They are very shiny though.