> and as far as I know, OpenSCAD is the only tool supported by the thingiverse customizer

I am sure you are right that this is still the case. This is a big plus for OpenSCAD.

One of the alternatives I mention takes customisation to a different level that makes me hopeful.

Replicad can be used as a library in a web app:

https://replicad.xyz/docs/use-as-a-library

And they have a demo here (a React app):

https://sample-app.replicad.xyz