DIY dos not necessarily reduce costs
you need to look very carefully at your mixing solution - it won't work - the frap tools mixing solution needs a minimum of 3 modules - for a single channel mixer - the module you have included in the rack is just the master - no channel inputs!!
I'd rip everything out and start again - put back in a single voice (a minimum viable synth) and work out how you would play, modulate effect and listen to that voice - think about how you will patch it - consider utility modules
then add another voice and do the same exercise - utility modules become more important
how do the voices interact? how will you cross modulate? how will you combine modulation sources to make more interesting modulation?
and then add another voice - repeating the same exercises... utility modules become even more important
you will probably need to replace a few modules along the way
you case should now be full - or nearly full - aim for 3 or 4 voices (max) in a 6u case is enough
"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