My best advice to you would be to download VCVRack (it's free) and set up a virtual Eurorack synth then try to play it.
In this set-up you're missing a lot of key elements to even a basic synth. What is a VCA? Why are they important and what can you do with them? Do you know the types of VCAs that are around? How many would you suggest for this set-up? What is an attenuator and/or an attenuverter?
How will you attenuate control voltages going to the resonance input for your VCF for example. The filter cut-off has a knob that functions as an attenuator... but the resonance doesn't. Manufacturers put attenuators and attenuverters (an added expense) on inputs they believe will get a lot of use. But there's no rules saying that they have to. If you want to control the amount of attenuation with other parts of your system, you'll need VCAs as well.
How will this synth play notes? There's nothing on here that can deliver a sequence of pitches AND gates. The Vari-Gate only does gates. What will handle pitch information?
You're also missing a lot of modulation sources. LFOs are very handy and standard sources of modulation. There's not one here. Where are the envelope generators? The Maths can do simple two stage envelopes... but what about ADSR envelopes?
Regarding the filter. It's only a low-pass filter. It has different outputs that supports different cut-off slopes. But that's the only kind of filtering you can do with it. I'd check out something like the Joranalogue Filter 8 to give yourself a lot more options... unless you're going to add a few other filter types to your system, which would be expensive.
The mixer is DC-coupled (so it can mix audio or it can mix control voltages). It's fine. But you may also want to consider an output module that converts synth level to line level. Modular synths operate at a much louder volume than your average line level synth.
Maths. Maths is a great utility module to have around. But it's a jack-of-all-trades.