Shake the damn things up for a start. You done one, you done them all. Its the same linear corridor of trash followed by a boss that most of the time carries no inherent difficulty. Lunar bahamut hits like a foam bat.
Shb bascially confirmed that dungeons with trust are basically overly long solo duties designed for afking dps. (walk up to trash, let tankcred pull, sit there for 5 minutes while the trust beats the enemy) where the only bit of work they have to do is occasionally walk out of a boss aoe and push a button to advance.

I get that they're set pieces, but set pieces aren't gameplay, they are pretty graphics. Make them a bit more random in the paths between bosses. Optional sidepaths with minibosses (and a buff after beating the dungeon so if you want to go tackle the miniboss but the tank barrelled straight past it and they have now left you can)