the first thing I would do is take the es9 and move it to the 3u - this will reduce your power consumption in the 6u by a considerable amount

don't forget that a lot of utility modules are passive - mixers, attenuators, mults, switches etc etc all of which are very useful in a modular synthesizer

as are blind panels - and they stop you dropping cables on to busboards etc!

"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia

Utility modules are the dull polish that makes the shiny modules actually shine!!!

sound sources < sound modifiers < modulation sources < utilities