We have a very similar use case. We want to take argument signature from parent class when we override the __init__ in subclasses. But we also want to extend the signature a little bit.
We asked if it’s possible to achieve this by adding a bound parameter to ParamSpec: Proposal: Add `bound` to `ParamSpec`
Seems the main issue is that currently we need a better way to express a function signature in a pure typing environment.