You've mostly dodged the #1 newbie mistake of "5 voices and no modulation sources" so that's good. Looooooots of modulation to be found here.
The main thing that stands out as missing is utilities: You have the one quad VCA, one mixer (though it's exponential so better for mixing audio than CV), and Maths which is a hybrid CV source/utility. Utilities let your modulation sources intermingle, giving you sounds that evolve in far more complex ways that any of those sources would give you on their own.
I'd look for what you're willing to sacrifice in order to put in another VCA module (perhaps a ring mod, so it's not just the same as the VCA you already have) and a DC-coupled mixer (for your CVs). It's a common refrain in modular synth: You can never have too many VCAs or Mixers. Some other utilities that may prove useful: Mults (Buffered Mults for pitch CV, passive mults can be replaced with stackable cables but sometimes it's nice to have an actula module for it). Logic modules. Attenuverters.
Consider replacing that RackBrute module with a powered case, or a backplane power supply. Power modules can sometimes be a little anemic (your setup is fine: only uses ~75% of the +12V rail), or sometimes couple power noise into the other modules.
Consider a "control surface" module. Something that lets you play the synth in a way other than "turn knob, push button". Joysticks, lightstrips, contact mics, ribbon controllers...Sometimes it's nice to take these very abstract instruments and make something about them a little more tactile. (A more complex MIDI-CV controller could inherit some of this from MIDI CCs, but that Behringer one pretty much only does CV-Gate)
Overall, it does look like a fun system to explore. Just giving you some ideas to chew on.