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!