Howdy and welcome to the modular madness!
The first and likely most important advice I can give is to expand to a bigger rack. I started the same way with the Tiptop Happy Endings kit, one row of 84hp with the uZeus, and I think I ended up getting a second row of 84hp rails within about 3 months. It sounds like you have a lot of things you want to try, and if you feel like your first explorations with your current setup has got you hooked, I'd recommend going straight for something like the Tiptop Mantis (absolutely best bang for buck in a ready made rack) or the Arturia Rackbrute.
Once you do that you have more room to explore different combinations of modules and see what piques your interest and helps you make the music you want to make.
Maths and the Doepfer VC mixer are great additions. Look up the Maths Illustrated Supplement. That will teach you A LOT about modular synthesis and voltage principles. I don't think you need the Behringer VCA as a final output, but VCAs are helpful for other things, like modulating modulation (maybe not as important yet since Maths is really your only modulation source, but likely worth holding onto). I've plugged my modular straight into a little Mackie mixer for years, and turning the gain down on the channel gives plenty of headroom so it doesn't distort. Not sure if your interface will do the same, but keeping the levels lower on the Doepfer mixer should be enough.
I appreciate that you're staying clear of smaller modules (way better for ergonomics), but I agree the 14hp Behringer VCO is pretty large even in a bigger rack. The Pony VCOs are pretty teeny and can be difficult to adjust manually, so things like the MCO, Make Noise STO, 8hp and up copies of Mutable Plaits and so forth would be great. Having two of a particular VCO is nice for detuned sounds or harmonizing in similar timbre spaces, so I'd recommend picking a VCO you like and getting two of them.
How are you using Clouds? If you haven't looked up the Parasites firmware, that makes the built in reverb way more useful as well as adding some additional delay and resonator modes. Highly recommended.
Other things I'd recommend: based on what you mentioned about melody/harmony/rhythm/randomness, a copy of Mutable Instruments Marbles is an amazing all-in-one sequencer/random source/trigger generator. Something like the Disting or the Ornament and Crime can also be super useful multi-function modules that can help you explore a lot of different types of modules and determine if you want a dedicated module to do the things you reach for the most in those resources. Once you incorporate more modulation sources, I'd also recommend something to mix them. I'm partial to the Tiptop MISO and the Doepfer Matrix Mixer. They let you turn a few simple modulation sources into a ton of different combinations and variations of more complex modulation. (EDIT Ornament and Crime with Hemispheres firmware is best for function exploration)
All in all, what you have is great for learning the initial signal flow of a mono synth, but the real fun of modular comes when you start to explore modulation. If you don't want to expand the rack, I think the Disting would be the thing I'd add. If you want to expand to a Mantis case, maybe swap out the Behringer VCO for a pair of other VCOs that you like, but definitely add a clone of Marbles, a Disting or an Ornament and Crime, a Tiptop MISO, and maybe a dedicated LFO or Sample and Hold from Doepfer or something.
Cheers!