Upping the spell slot limit from 24 to 26 would future proof BLU and still have the need to make sacrifices among what spells you include in a set.

Make Mimicry Crystals a thing. You could buy crystals with poetics that instantly give you the mimicry (one crystal for each mimicry type). The spell would remain of course as a free alternative- this is just a quality-of-life thing and another way to spend some poetics, but the price has to be reasonable (somewhere between 30 and 60 poetics) otherwise I would just keep using the spell. Whether or not the mimicry crystals should be useable inside an instance, I'm not sure (probably not).

Give BLU a triple cast spell, but make the spell itself have a cast time accordingly. The triple cast would give you at least 20 seconds to do the 3 casts. Not fussed about what the recast time would be. 'Ability' spells would not steal away one of the active instacasts when using them of course.