Could it be possible to add an attribut for both stamina and stamina recovery? It would improve its integration with other mods that add attribut bonus like armor tiers and so on.
If it's already on I just don't find these with /attribute

Implemented in 21.x.

I made a mod that allows this on fabric, on forge theres the betterparagliders mod, if you can get me in contact with the mod owner of betterparagliders we could distribute the mod, its a port of his mod

In the upcoming 1.21.x ports I'll add max stamina attributes, alongside with other changes like changing stamina value to double.

Modification to stamina delta is a bit tricky, since it is largely dictated by movement states, and whatever systems it might be, should be able to factor them in. The full spec is not yet determined, but I will consider implementing bunch of generic delta-modifier attributes on base mod.