Looking for some advice and alternative suggestions on how to fill out a 60hp Moog skiff to serve as a utility box for my existing group of synths. I'm mostly interested in making offbeat sounding pop music and Mort Garson-esque instrumental noodling songs.

ModularGrid Rack

After a lot of lurking I've noticed a few trends over and over which I am trying to be conscious of. First, everyone wants to make a minimal modular box because of youtubers that is more good looking than functional and has no room to grow. Second, everyone fills their cases with a bunch of fancy modules and forgets to add the connective tissue. Right now I have a group of synths I'm very happy with including a Korg Odyssey reissue (modded by a previous owner to include CV outs for square and saw waves), a Microfreak, an OP-Z driving everything as a drum machine and sequencer and the most recent additions: a Microvolt 3900 and a DFAM. These last two have got me a little curious about modular but I am not interested in building anything from scratch at present or diving headfirst into a big build. However I have noticed some deficiencies in the two semis and am looking to correct/enhance them via a small utility skiff. I like the Moog 60hp skiffs for vanity reasons because they look nice and also because I have limited desk space and I can slap one (or eventually maybe two) onto my DFAM via the Moog brackets. I am NOT interested right now in expanding into Eurorack in a big way or having this be my "starter kit." I am purely looking at these modules as adding additional functionality to my existing synths, which is why I'm not super interested in getting a massive case. I don't think I will be filling up much more than this.

My thinking for picking these modules is as follows:

1) the Microvolt has a lot of waveforms that it can spit out but it quickly runs out of ways to do that internally. At best you can coax two separate ones out via the split VCA inputs inside it. Adding the Quad VCA gives me more options for amplifying, though I am worried it might be overkill.

2)Similarly, the Maths gives me a lot of additional possibilities for mangling and changing the signals coming out of both synths. This seems like the "no brainer" cliché module but also one of the ones that has me very curious.

3) I added the Behringer 140 to get some more envelopes and another LFO at a cheap price, but I think it is maybe taking up too much space in the skiff? I'm not married to this one at all.

4) the DFAM sounds amazing but could use some quantizing to make it more melodic and able to play well with the other synths, which is why I added the 2HP tune. I know this is also available in the Disting but I thought that could serve better purposes in other areas, like an additional S&H, and I think I would be quantizing the DFAM enough that having a dedicated quantizer is worth it.

5) if I'm being honest I added the Buff because I had 2HP left over and it seemed like a nice thing to have.

I would probably buy the case to start and then slowly fill it with these, starting with the quantizer and Maths. There are so many threads on here of people's tiny cases and I recognize that lmao. In this situation I am really trying to specifically complement my existing studio set up rather than set off to build a big rack.

Seems like you have done your research and you are definitely on the right track. With something this small, you will probably want to pack as much functionality as possible into the small space without sacrificing playability and fun. There's a trade-off between tiny modules and the fun factor, so a balance of large and small will maximize functionality and ergonomics. You also will probably want to consider a few function-dense menu-driven modules that detract from the one knob one function approach, but give you lots of options.
Maths and Disting are almost "must haves" in your case, from my perspective anyway. You can do the subharmonicon patch with Maths and DFAM (see Loopop's video). Disting does a bit of everything, though only one at a time, so you can add oscillators, filters, effects, waveshaping, utilities, etc. depending on your needs. You have attenuverters included in Maths too.
Veils and Kinks are probably the route I would go for VCAs, logic, and S&H. Pons Asinorum is a cool multi-envelope generator/LFO in a small package. Pam's New Workout can do all kinds of neat clocked things.
Take a look at how I might approach your specific build below. I'm sure others may chime in with other ideas.
Have fun and good luck.
ModularGrid Rack

*Edit - Keep your mults outside of the case with those inexpensive little star-shaped external mults you can find at most modular retailers. You probably won't need a buffered mult at this point.

Awesome, thanks! These are some great places to start.

More generally for everyone: one other thing I forgot to ask about is that I know the Moog cases are pretty shallow, they apparently go to around 48mm, and I know some of the modules, especially the 2HP ones can be very deep. The Tune, for example, is listed at 47mm so how tight are we talking about here. This would apparently TECHNICALY fit but...

Well, here's where the "you need a bigger case" conversation starts. Haha
As you know, you are indeed limiting yourself from the start. Many of the tiny palette style cases are shallow, and so you are limiting the number of modules you can choose from. Once you factor in all of the ribbon power cables and flying bus cable, you very well may find that some of your modules won't fit (or won't fit where you want to place them). Also, limiting is the rack-wart power supply eating up valuable rack space... so, the choice is yours.
I know the aesthetics of the Moog case are tempting. An alternate consideration would be the 104hp Make Noise powered skiff. The power supply is built-in and plenty powerful, it's deeper (but not super deep), and you have a broader range of modules that you can choose from, depth- and hp-wise. Something to consider, anyway.
You are stepping into a world that offers a mind-boggling potential for sound sculpting, so I recommend taking advantage of that without placing any unnecessary limitations on yourself from the outset.
Have fun!

Fun alternates for Maths would be Stages or Zadar.

I'd start with Pam's, Zadar and Disting. Then add a wicked effect module like Mimeophon, Beads, Nebulae or Arbhar.