Thanks for your suggestion. While I don’t think it makes sense to not trust system-wide CAs on app level (if you distrust those CAs, wouldn’t they have to be removed/disabled in the system so that they can’t do any harm in other apps, too?), I certainly support that certificate pinning would be a good thing.
Thanks for the hint, I’ve added it to the roadmap for DAVx⁵:
As you can see, there are a lot of possible enhancements, so maybe this will take while…