Most heirlooms only work in a limited level range in their default form and must be upgraded to work at higher levels. That costs gold, and you’ll need around 30,000 gold to outfit yourself with a fully upgraded set (counting only those that give you bonus experience). You may need to spend some time on a boosted character to acquire the gold you need to outfit another character with heirlooms, but it’s well worth the effort.
Unlock all of the traits in your Azerite Armor. Each piece of Azerite Armor — which can be head pieces, chest pieces and shoulders — has three traits that you can unlocked with Azerite Power, or AP. AP is something that most types of endgame activities will give you in Battle for Azeroth, but some are more effective than others. In the first few weeks, you’ll want enough AP to unlock all of the traits on all of your armor, however much that may be.
For reference, my previous best time abusing Method-0 in 7.3.5 was 32.4 hours(as show in the old thread). A difference of just over 8 hours; which isn't all that bad, all things considered. And I believe I made several errors along the way, which could be cleaned up in future runs. I believe that that gap could be significantly closed by using a monk's bonus, but I hate playing monk...so...yeah. YMMV