A few other things I discovered and that you might want to mention somewhere: Scala 3 typeclass derivation doesn't support a few things that Magnolia supported with Scala 2:
value classes
nested sealed traits
Opaque types also don't have mirrors.
And I think default values can't be obtained from the macro yet.
I suppose a good idea here would be to move all of the magnolia-scala2 tests into magnolia-scala3, and see which ones fail. Mark them as such, but keep them around as a good documentation for further work.
A few other things I discovered and that you might want to mention somewhere: Scala 3 typeclass derivation doesn't support a few things that Magnolia supported with Scala 2:
Opaque types also don't have mirrors. And I think default values can't be obtained from the macro yet.