Solution simple: an outboard mixer. It feels like you're trying to get way too much out of the internal mixing strategy, and with all of the other devices, trying to run them through the TexMix is just more complexity added to complexity. Plus, running external equipment then feeding it to the modular's mixer (which is expecting modular-level voltages) probably won't sound all that good in the end. Get your audio summing out of the cab, and then you automatically solve the "drums in cue bus" problem.
Next...unless you've got an Arturia Minibrute 2 or 2s, I don't know why you would use a Rackbrute. They're nice and all, but they restrict you to the Minibrute 2's form factor. Plus...well, looking at Sweetwater here, the 6U Rackbrute goes for $359 at two rows of 88 or 89 hp, depending on who you're listening to. And right next to it is Tiptop's Mantis: $335 for 2 x 104 rows. And if the expandability of the cabs is a future concern, the Mantis can link with a second Mantis via a linking bracket to give you a crazy AF total of 416 hp for $670. Beat THAT deal!