Why not make use of the transmute system, but add an option to reconstitute select gear for companions.
Select gear being "only passive effects" so most sets won't have the 5 piece bonus since it's usually a conditional effect, but some pieces like beekeeper's gear will since the 5th piece is health recovery.
Max mag/stam provide a boost to abilities, mag/stam recovery act as cooldown reduction.
It gets people out in the world and content looking for gear to bind to their account since a bunch of previously underpowered sets now have value, as well as transmute crystal hunting. Would even get some people in normal trials for the 3 piece minor aegis/slayer.
Then to make it even easier on the devs side of things, add an item that lets a companion go past level 20 that is either rare or for crowns. Like 1 per completion of Caldwells gold or for crowns. Level 20 companions use the current system, the select handful of level of promoted companions use transmute station gear once they're level 50 or 50CP160.