okish - some thoughts:

needs more utilities to get the most out of the 'expensive shiny modules'

more mixers - a simple cheapish mixer would add the ability to mix 2 vcos (or in the case of the doepfer 2 or more waveforms) before hitting a filter

attenuators - always useful and inexpensive

another modulation source is a great idea - to complement maths - which is fantastic but really should be seen as maths - see 'maths illustrated supplement' - it's a great jumping of point and learning tool for patch-programming and patching in general - but another modulation source would add a lot - especially when combined with a matrix mixer

possibly also an envelope generator - an adsr if you want to sound like a keyboard - or something less complex if you're not so bothered

end of chain mixer - it might be a better idea to get a bigger one now (so you are not selling this one to get a bigger one in the future) - there are also expandable mixers - at the lower end of the market - tesseract tex-mix is good value for money - nb sidechaining/ducking can be easily patched by mixing the envelope of the sound source you want to duck and an inverted copy of the envelope of the sound source you want to sidechain and then use that into the cv input of the vca you're sending your

I'd go for the fx aid xl over the milky way - it's more versatile

for the utilities - doepfer is a great brand - inexpensive and reliable - also really good for other things like classic filter designs and vcos etc

"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia

Utility modules are the dull polish that makes the shiny modules actually shine!!!

sound sources < sound modifiers < modulation sources < utilities