what should i fill with the last 20 hp? i was thinking maybe erbe-verb but im combining this system with DAW so effects is not necessarily big deal
looking forward to hear your ideas
Intellijel Quad VCA and Tiptop MISO. Boring on paper but super fun in practice. I also have a little Buchla based setup and modulation/expression/movement is key here.
If you have a DAW and some money to spend, though, you might also look at the Expert Sleepers ES-8 or ES-9 interfaces for interfacing your DAW and modular. Especially in tandem with VCV Rack. It has DC-coupled inputs and outputs so you can sent control voltage to and from your computer, thus allowing you to use software and hardware modules in the same context. This means your setup can expand indefinitely without you spending much money at all, relatively speaking.
Even if you do this, though, I still recommend some kind of hardware attenuation or VCA just to make things playable. The Doepfer A-133-2 is a good small option. 2hp has the Avert and VCA modules, but be careful with these - you have to make sure you have a place for them where they won't get buried and inaccessible.
It repeats some of the more conventional functionality of Maths, yes, but this frees your Maths up to do other interesting tasks like envelope generator, slew limiter, (low or high frequency) oscillator, subharmonics generator, envelope follower, filter (or just gives you more of the vital functions they share to control your modulations). You aren't really missing functions and tools in this setup - it's more about plumbing to ensure what you have does what you need.
Looking at this system, another idea is one of those 2hp passive low pass gate modules like the Meng Qi DPLPG or the Takaab 2LPG (which is almost better because it has a switch to change between no filter, some filter, and more filter, making it good for transitions or CV). I know you have the Optomix, and that one is great, but I have a 266t and those noise sources are great for percussion or weird sounds to send to resonant filters. It's an ungodly cheap and compact way to add more signal paths into your setup.
Frankly, blanks could also work at this point to create space between some of the smaller and denser modules for ergonomic purposes. Like all of my suggestions, really, this is based on the fact that what you have is already a very deep and powerful system that, supplemented with computer material, can provide you anything from drums and percussion to 60s art noise to pretty twinkly melodies and be very productive. I wouldn't add anything much more complicated to this and a computer (and whatever else you use, I guess) before stopping to practice, record (ABR - always be recording), make some songs with it, assess, and see how much progress you've made so far. Ultimately, you and your responses to what it does will be the best guide.