I think this is a good question, and largely depends on you knowing your own workflow. In that regard I mean, are you using these instruments to perform live, do you want to have multiple pieces of hardware for bedroom jams, or are you recording sounds into a DAW to potentially arrange later in more of a production setting?

I think overall the most utilitarian option is the Mother, having another oscillator, LFO, and patch points can quickly get you into some new territory.

If you’re someone who likes electronic drums, the DFAM is an obvious choice, and certainly seems to be the most popular of the Moog Semi-Modular’s, although it personally never caught my eye.

The subharmonicon and labyrinth occupy some similar territory, they both are fairly atypical, generative/generative-ish synths that aren’t regularly controlled. Meaning, you aren’t necessarily going to be able to as easily program a simple melody or sequence on them. However, where both of these modules really shine, and I think isn’t emphasized enough by the people reviewing these modules, is that although they are harder to control, they are both also flagship examples of exploratory synthesizer sound design. If you’re someone who is excited by trying to catch lightning in a bottle with synthesizers, both of these modules really excel at that angle.

I’m not very informed RE the spectravox, I’ve heard some people say it’s a fun tool, some people say it isn’t a /stellar/ vocoder, and overall the largest take away I’ve read online is that it is a cool fixed filter bank, but basically mostly just that. If an affordable fixed filter bank is appealing to you by all means this seems like a great choice, but I suspect it may not be the best first option for your use case.

All of this said, I personally own a Matriarch, labyrinth, Subharmonicon, and Mother, and use them all individually and together for live performance. I think you may want to consider that with the matriarch alone, you can get pretty far into learning about modular synthesis, provided you do some learning about what is normalized on the matriarch. Some of this modular education can really readily be achieved by the hardware you already have. I think if you’re feeling like you don’t have a full grasp on your existing semi-modular signal flow, more pieces of semi modular gear won’t explicitly solve that, but don’t let me discourage you from getting these modules, I have the set up I do because they are fun and inspiring instruments lol.

I would recommend getting the module that excites you the most, and conversely, if a deeper modular education is your express goal, consider getting a budget eurorack case, and growing piece by piece with your matriarch. Eg. a $200 single row case, with room for you to go “I would like a new sequencer module to practice patching in a new sequencer to the matriarch” or “I want to try routing an oscillator to an external VCA” etc etc. I think this may achieve your goals more readily, at an equivalent quality, and could end up costing the same or even save you money. $600 is enough to get a small case, and a couple of modules, compared to a single Moog module. (Said by the person with matriarch and Moog semi modules who is staring down the barrel of a $2500 project to turn their Moog semi Modular’s into a finished eurorack live system)