When you complete main scenario quest "shadowbringers" the trusts can then be leveled in the shb dungeons starting at level 71 with "the Holminster switch" and again with the completion of "endwalker" you'll then be able to start leveling the trusts from 81-90, starting with the Tower of Zot. It takes about 3 or 4 runs per level. The grind is real to level them all lol.
You'll be running each dungeon at least 10 times to level all the npcs. It really is quite handy collecting the gear sets and it can be relaxing to just keep going with no wait time.