What's missing here isn't some magic module -- it's SPACE. You're trying to cram two different machines in a space that's really suited for only one. As a result, there are A LOT of things missing...VCAs, attenuators, FILTERS (good lord!), and so on. This is a poster child build for "Sexy Module Syndrome", to be sure.
First of all, get rid of this idea that you can cram a proper synth voice AND a full-on drum machine into a 6U Rackbrute. Ain't happening...at least, not with these huge spacehog modules. And this statement: "I considered the Arturia Minibrute 2S but I don't really need the synth engine there although I could be swayed if anyone has an opinion on it"...uh, no. Right now, if you intend on sticking with this build (which I do not advocate), all of the things you need to make this work are in the MiniBrute.
There's absolutely no way you can do proper "creative sound design" with the bottom row as it stands. There's far too much missing...there's not even a "voice" there, tbh. Delete this, start over...and the next time around, decide if you want drums or (NOT "and") a synth voice in the Rackbrute first and then build that out. And before building, study some other builds that have been done right to see what they require to do what they do. Otherwise, sticking with this sort of plan will result in building a black hole that sucks in money...but which puts out nothing useful in return.