To get the most out of Metropolix, you'll want two pitched sound sources (i.e. VCOs). I only see the Manis Iteritas. You could use the Disting as a VCO, so that's an option, but you might want to use the Disting for other stuff. My point is, think through how you want to use the two sequencer tracks on the Metropolix. A dual oscillator like Twin Waves pairs really well with it IMO.
Not having a dedicated VCA... yeah seems like a problem. I assume you want to generate envelopes with Quadrax and not just use it for LFOs? How are you going to apply those envelopes? If you drive two VCOs with Metropolix, you'll often want the Metropolix gates to trigger two envelopes that are applied to those voices with a couple VCAs. Manis Iteritas has a built in envelope, so you technically don't need a VCA for it, but you might want to use Quadrax instead sometimes to get different envelope shapes (for example a burst envelope). And if you decide to get a second VCO for Metropolix, it might not have a built in envelope.
I think Steppy is good for drum triggering and will let you sequence up to four drum sounds. I see three dedicated drum modules. You could use the Disting for a fourth drum (via samples using the wave player algorithm) or you could use the Disting to combine two Steppy tracks with logic for more interesting rhythms. So you kind of have that covered but you might consider another dedicated drum sound source, or something more general/modular that could make drum sounds: If you had a source of white noise and a VCA, you can make hat sounds with a fast envelope from Quadrax. It's fun to have some different options like that so you can change things up from patch to patch.
The running theme of all my feedback so far is: Think about the two voices you can control with Metropolix + the four drum sounds with Steppy and plan for how you are going to patch it end-to-end including envelopes and mixing. If you end up with six total sound sources, you need to be able to mix them all together. Personally I'd go for a quad VCA that can mix four signals together and have another dedicated mixer.
A few other thoughts:
You have a stereo mixer but no real source of stereo except that reverb, so the stereo mixer seems like a bit of a waste in a rack this size. You don't have a way to pan anything in the stereo field (again, the Disting could but you'll probably want it to do other stuff). You might want to focus on mono for now. Or for basic stereo you could get something like a Doepfer A-138s Mini Stereo Mixer and pan your drum sounds to different places in the stereo field.
If you focus on mono, you can probably ditch that reverb and get a simpler mono reverb
I like to change up FX, so I'd be looking at a multi-FX module instead of a dedicated reverb. Maybe you love reverb, so do whatever you want there. Dedicated FX modules often have more extensive modulation capabilities so that might be a reason to avoid multi-FX.
The Golden Master seems like overkill in a rack this size. It has mid-side processing but as I was saying, you can't do a lot of stereo processing in this rack. Unless you really like squashing your sounds, you don't really need a compressor/limiter. And personally I would use filters over EQ in a modular rack, so I would opt for a second filter if it can fit.
How are you getting audio out of this rack? I think a dedicated output module is a good idea. Technically you can run your final output directly to an external mixer or audio interface, but it will be much louder than the other equipment expects so you need to be really careful about your levels if you don't use a dedicated output module (always start with the volume all the way down!). Read up on this: https://www.perfectcircuit.com/signal/eurorack-line-level
And I'll leave you with the usual advice: If you are planning to fill this whole rack up relatively quickly, you need a bigger rack. Once you've spent time with this rack, you are going to get new ideas and want to expand. Unless you are sticking to a tight budget and are very disciplined, you'll end up running out of space and end up spending even more money (trust me, I know). Of course, a second rack is always an option but I think it ends up costing more in the long run. It's also a good idea to go slow: start with just enough modules to make some sound and slowly add more when you are sure you want the additional features. The Disting is a good way to try things out before you buy dedicated modules.
-- adamj
Adam, thank you so much for that detailed analysis. It has brought me much insight with respect to what should I ask myself in order to optimise it. I will also go for the 7U performance case as well.
I noticed that you seem to have quite a lot of experience with the Disting. I often see o_C being mentioned as having a similar use case, how do both of them compare? Is it worth considering the 4Robots plum audio module over the golden master, and get the intellijel line out?