Maths - it's a good learning tool - what with all the video resources and the 'maths illustrated supplement' - work your way through the latter a few times concentrating on what, how and why and you'll do yourself a big favour... & it's also incredibly useful and not small...
A matrix mixer - incredibly useful - from taking multiple modulation sources and making them more complex to setting up send/return or feedback processing for effects
xaoc samara would also make a good addition... as would a kinks clone
I'll point you to my signature - as I usually do in most posts - for a few hints on how to get a versatile modular for as little as possible - the formula scales from tiny systems, to the truly massive
I only see one uZeus module - and the power draw is over it's capabilities - on the +12v this can be remedied by using the higher powered 'turbo' power adapter, but the -12v is probably quite bit over... not only should you leave at least 20%, better still 30%, headroom on each rail, but 533mA is more than the max draw on this rail even with the jumper switched - regular setting is just 400mA (so you don't really want to be going much over 300mA - don't skrimp on power - it's really important!!!
all this of course can be remedied by removing the West Pest...
how are you playing this? a sequencer might be a good idea... at least 2 channels...
how are you listenimg? via an external mixer or audio interface?
"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