A couple of thoughts which may or may not be helpful, but as this case is very similar to my 60hp ones I build it might be useful to you. I have a goal in mind when putting modules into the case so that there is some method to the madness. As your goal is to make ambient let's explore how to adapt this a little:
Pams + Ornament and Crime is your killer power combo and will probably do away with Maths, freeing up a ton of space. Pams is going to give you all sort of rhythmic clocking options, and LFOs and euclidean sequencing. Plus it can be modulated over CV itself. Pams will then clock OC. The stock firmware in OC gives you lots of power; Sequins is a dual 16 step sequencer/quantiser/arpeggiator so you can drive Plaits use the other sequencer as a S&H to modulate stuff (e.g. the filter on Ripples). For generative stuff that's quantised to a scale you can use Copiermaschine - a 4 stage cascading delay shift register. I've made some tutorials on it if you're interested.
I know everyone says 'get Maths' but you don't have to as there are lots of other ways to achieve the same outcome in the modular world.
With all that you can get rid of Scales. Ornament and Crime does quantisation and if you go the Hemisphere's route on OC you get even more quantisation. Pams also does quantisation when generating melodies.
Then I'd think about ambient; lots of delays that are modulated, reverb, filter, shimmers etc. You need something to bring that sound to life with lots of modulation. I use a Happy Nerding FX Aid XL (6hp effects powerhouse) or a Mimeophon (a lot bigger but insane for ambient washes of sound). Throw some modulation into either of these from Pams and you're already half-way to the races. That also means you can drop the 2hp delay and verb modules as the FX Aid does far far more than these two can and you can re-configure it with 32 algorithms of your choosing from a much larger set.
Doepfer VCA is a solid choice and lets you modulate the intensity of your modulation signals using LFOs from Pams or randomness from OC. Plaits and Ripples both have a VCA built in so you can use that with an envelope to shape the sound. Envelopes from Pams are a little limited in shape, but the Piqued app in OC gives you lots of envelope-y goodness.
I think you might want to think about some hands-on controls for Pams and OC as they are more menu-driven than hands-on. It's easy enough to buy some voltage sources to control them with though. Another alternative, and much loved by ambient folks, is Marbles both as a source of generative randomness and gates but also for its playability.
You might consider some CV mangling units to give you a few options for attenuating/mixing. I use the Befaco A*B+C for all sorts of little jobs. And you'll never go wrong with a Disting Mk4; it can give you another synth voice, play sample loops (how about sounds from nature playing on loop while you jam over them with Plaits into Mimeophon to be looped and warped anyone??), be a filter, mixer or even an LFO/S&H for you. You name it, Disting can do it.
Also do you have an audio output stage to hook your modular into a mixer or listen on headphones? Most mixers can take modular level audio signals but you might enjoy plugging in some headphones to take your ambient case out into nature and jam in the forrest (improves the sound 10x I'm told)
Have fun!
-- synthdadmusic
Thank you so much for your comment! This is really good advice, and I think I'll definitely get Pam's. Also, yes I have utilities in the case itself (Nifty Case) for things like MIDI to CV and headphone out.