Howdy!
I apologize, I just realized in my earlier response I recommended getting the Behringer 2500 clone, and I meant the 2600! Looking at your proposed rack, the 2600 covers pretty much everything the Sytem 100 clone modules would do but in a more economical package. You can find a Behringer 2600 for about $400 USD used. I arranged this pair of 84hp rails as an accompaniament to the 2600.
I kept pretty much everything you had in your proposal less the Scales. This felt not totally necessary as the Chaos can output quantized pitch voltage, and since you're also using a keyboard sequencer the scales felt a little overkill. I also removed the Radar as the 2600 has an input/preamp/envelope follower built in. I added a Clouds clone as the delay/granular/reverb effects in that just make for an instant, easy, and satisfying ambient texture, whatever you want to throw into it. I also added the Behringer copy of the Intellijel Quad VCA as this is super helpful for both standard VCA duties as well as mixing signals, both audio and CV.
I will say, I know the Behringer modules are cheap, but there are definitely some ethical considerations with their practices in the Eurorack space. I have less concern with their clones of older/hard to get synths like the 2600 (and I'm really pretty pumped to see their Buchla Easel and EMS Synthi copies if those ever see the light of day), but some of their clones in eurorack feel a little dirty. If you can save up a little more, I think it's worth it to get an actual Make Noise Maths and getting Plaits, Marbles, and Veils clones from After Later Audio or Cal Synth to replace the Brains, Chaos, and Quad VCA respectively.
But as is, if you go with your proposed setup, I'd just recommend dropping the Scales and adding a cascading quad VCA like the Behringer Four Play (or the Intellijel Quad VCA it's based on, or a MI Veils clone) and a more robust delay/reverb/effect like a Clouds clone. I also think the Behringer 2600 is pretty much the best bang for buck intro to (semi)modular synths out there. I love the joystick and ribbon combo, that'll make for some really interesting human modulation, and if you pair that with a VCA to control other modulations, you'll have a nice stew goin'
Cheers friend!