great system btw!
Metron and Metropolix both have probability, pams has euclidean, zadar has bursts, cara has controlled loopable random (botb cv and gate), batumi can self modulate. think up some processes that excite you! Use mixers, mults and attenuation you already have to glue your modulation together! matrix mixer is a great idea if you must buy something. but looks complete already to me!
monorail tech talk is a great youtube resource to think better with ingredients.

if the size of your system creates overwhelm, throw 5-6 modules in a smaller case to focus on how to wring them out.

EDITED Jan 11th, 07:10

you have so much modulation in there! just stare at your system and read up in how to use it as like you'd like. $0 answer!
pams, metropolix, maths and cara alone will get great results. assume you have everything you could possibly need.

:)
sequencers- everyone is going to give you a different answer.
a beatstep (pro?) or keystep is an ol' reliable for out of rack sequencing.

Peaks is a good idea, it packs a lot. just be sure you learn the dead mans catch firmware really well, i sometimes get lost not knowing what mode im in. i have a piques which is a 4hp clone. its very tight but saves space. Klavis Quadigy might be a consideration for envelopes if you like sliders. its quad, can loop as lfo's.

the effects suggestion above from zacksname is worth considering. ambient generally implies heaps of reverb which is an end of chain effect usually, it doesnt have to be in the rack. a used strymon bigsky or something might treat you.

my 2 effects suggestions for in rack are a mini clouds clone (called a uburst) or an fx aid or fx aid xl (not the pro, too big). a typhoon is a deluxe clouds with sliders if that sounds interesting, maybe too big though for now. clouds has traditionally been kind of a rite of passage for ambient modular, but dont let that sell you (who cares lol). the fx aid is open source, and you choose what effects go on it from a web app. the list is quite good. both modules are 6hp or less.

have fun!

EDITED Mar 16th 2025, 07:36

Maybe a doepfer quad LFO in 4hp, and an intellijel Quadrax (which does lfo’s and envelopes)
I mention the quadrax because that dual adsr is a space hog in a mini case. I’d also take out the joystick just only to make space, since the use case is limited for the hp it takes up.
I’d personally specialize the case to be either one or the other of your intended ideas. It’s totally possible to do both, but you’ll probably have to do some trial and error before arriving at what you know you want. It will be easier and more inspiring to nail one use case instead of half loving a dual purpose case that doesn’t do either side well. Maybe the restriction will be freeing for you.
Have fun!

I second that you should take out voices for an FX rack- if that's your intended purpose.
An unsolicited opinion for an addition that's hp- economic, the FX AID XL is a huge bang for the buck (6hp), though they might be hard to find new.