A larger set of options for skills would be really nice to have. Or perhaps a write-up on how the current skills like mining_speed work and how additional effects might be added, so motivated devs can make PRs to add additional skills to this mod?

Or suggestions on built-in attributes in forge, fabric, or common mods?

Trying to figure out how to add new definitions for other skills.

Maybe forge's atribute system or apotheosis

You can just use forge & apotheosis attributes directly and make your own skills with those. No special support needed afaik

In my case it's Fabric, which doesn't have as many attributes exposed.