You can’t get into 10s or heroic raids at 300 item level chief. Groups require 425-430 for that unless you get a carry from friends or buy one.
OP if I had a fresh 120 I would go straight to naz and get a full benthic set at 385. Go to the ah and spend a little gold buying the craftable pieces that aren’t available from the benthic vendor, such as weapons and rings and maybe a trinket if there was a cheap one. Your alts should have enough manapearls to send your new guy a full set. If not, then you can farm enough pearls in a couple hours doing the quests and killing a couple rares.
The reason for doing this first is because the gear that drops from pvp and world quest rewards scale based off your current item level. You would easily be around 370-375 item level in 5-10minutes versus 5 days.
After you’ve got your base set then do your world quests while trying to get your daily bg & brawl win. Every day you should be trying to get a daily win for the bonus conquest. You should also try to catch supply drops while farming your manapearls in naz and a supply drop in the regular zones doing your world quests. These give you gear and conquest.
You can upgrade your benthic gear to 425. Pull up the the pvp conquest gear schedule on google so you can see what pieces are coming up soon. Focus on upgrading the benthic pieces that you won’t replace with conquest.
When you get to 410-415 start farming m+. Start a group for your own keystone. Stack your team with other classes that have a higher item level than you and wear the same gear type. This increases the chance they will not need an upgrade and will trade it to you at the end of the dungeon. For example if you are a druid, try to only invite rogue/druid/dh/monk to your groups all with a higher item level than you. Once you get rolling on this strat, this is by far the fastest way to go from 410 to 440.
Once you get to 430 your life becomes infinitely easier with LFG.