Well, first up, Intellijel's Noise Tools would also be a good contender for that tile row; you might consider dropping the USB for that, as you have a backside USB on the Intellijel 7U case. Also, I'd invert the rack...put the utility tiles + your I/O on the bottom row to get the external cables out of the way. Consider, also, finding a way to squeeze either a Digiverb or a Pedal I/O in there.
Ambient, from my experience, requires quite a bit of heavy-duty processing to get the right flavor. The Orthagonal ER-301 would be a good multi-possibility choice there and would likely allow you to take out the Clouds, which Mutable discontinued recently (someone needs to edit its listing). If you have to have the Clouds, your best bet (space saving, too) would be an 8 hp third-party version of the module, for which there's several sources right now.
Nothing here in terms of modulation sources, especially slow-duration ones. The MATHS is a good choice, plus adding a Mutable Tides would give you plenty of slow-duration modulation change with good control capabilities. And VCAs...got to have those. Intellijel's Quad VCA would play nicely in here, plus give you a 4-channel mixer for either CVs or audio in the process. You might actually consider two of those, one for each.
Two VCOs, also...not really enough. The modular I currently use for this sort of thing has three VCOs plus a VCDO, Digisound's steppable fixed-wavetable oscillator. You're going to want some spectral complexity that you're clearly moving toward, but still a good ways away from at this point. You might get closer by yanking the Just Friends and looking instead at Doepfer's Quad VCO, which just came out a bit ago; I would definitely keep the Mangroves, however, because it would provide a nice timbral contrast to the Doepfer VCOs.
Keep in mind that with ambient work, complexity is key...you want slow change with lots of possibilities. You might even consider going to a second cab here so you can have more sonic possibility while staying with the Intellijel format tiles. With the simultaneous use of a Seaboard + Monome setup, something this small-scale might actually wind up limiting the full capabilities of those controllers in tandem. But with a dual-cab rig, you can use the Monome with one for functions that're optimized for it in case #1, and the Seaboard in a likewise manner for case #2, sort of like a dual-manual for an organ...but not an organ. Means more monetary outlay, sure, but you get that back in terms of being able to make more complex, subtile, nuanced music with the instrument. Sure, it opens another can of worms, but it's a can with more potential.