This guide is legendary. It helped me to learn and follow the process, and I made some pretty successful parts. For example, my own keycaps for all keyboards that I use: much larger than usual (for my large fingers), with a pleasant gently matte texture, in beautiful colors.

Polyurethane resins are an amazing achievement, and very much underrated. So are platinum-cure silicones. With care, you can get design-to-parts precision of ±25μm, which is spectacular (and a bit surprising, too). The fact that modern polyurethane resins (Sika Biresin F50) have essentially zero shrink helps quite a bit, too.

Incidentally, there is a whole bunch of youtubers doing casting using epoxy resins, or cheap silicones, there is a large following, but this is not representative of what the techniques really allow.

If you want to step up from 3d-printing, this is the way to go! Especially given the proliferation of inexpensive desktop CNCs with really good precision (Makera and others).