You have to do the intro quests for Nazjatar, Mechagon and the Heart Forge on every single character, no skipping.
It is unbelievably tedious for somebody who plays 10+ alts, such as myself, but I do have a few points of advice.
Number 1: Unlock Nazjatar and the Heart Forge on all of your alts as this is required for them to be relevant in any sort of end game stuff (annoying, I know) but this doesn’t mean you need to grind Nazjatar WQ’s or dailies on them, just unlock the Heart Forge and do the dailies which give an essence.
Number 2: Only actively do Mechagon content on one character, doing it on multiple will take you too much time because quite frankly the zone is poorly designed gameplay, content and progression wise.
Number 3: If your alts have trinkets above 400, ignore Mechagon on them entirely (they will not be missing out or falling behind if you do) if they don’t have 400+ trinkets, do the intro quests to Mechagon and do the 400 trinket quest and then leave the zone and never return on those characters.
Number 4: Use your main to grind low level Benthic gear for your undergeared alts, so they have a chance at completing the awful Nazjatar introduction quests.
This patch is extremely alt unfriendly, and to all those who could reply to this and give me crap about how you can buy benthic gear for any alt and gear them up quickly blah blah blah, yeah, you can do that, but it is completely obsolete 385 gear and in order to upgrade it you have to run around a zone doing tedious and boring world quests on multiple characters because for some reason you can’t trade the new currency.