I have had my eye on that FX AID too, I'm mostly always going for the keener priced options.
I also want to get the 8NU8R from Malekko Heavy Industry as it does Rectification, I could easily mult and i can mix and attenuate in the DPW AV-1. I've been suggested the Happy Nerding 3xMIA which looks like a good option for further LFO mixing, though if i didn't need Befaco STMIX for audio I could use that for blending some LFOs.
Good call on the AI Matrix Mixer, and its a DIY thats my sort of PCB density for my sanity when soldering!
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.