I ended up using one of my more generic domains (https://www.modulecollective.com/) and then launching things as <product>.modulecollective.com. It's a little wordy but it's free and creates kind of a natural place to catalog things. I figure if any of them take off then I can go buy a better name later and move it over, having the old one redirect. That would be a good problem to have :-)

That particular domain was going to be like a Netflix DVD style subscription product for Eurorack modules but I never even ended up trying to build that after buying the domain.

I think this is perfectly valid. Why waste the resources until you've proven it out?