Your data is encrypted locally with Proton. Your second sentence is what you really mean, and I’m not saying you have to use or trust Proton, just that because of that local encryption of the data, third party apps can’t access the data without compromising the security of the service.

Your described setup takes knowledge (and patience!) which customers of Proton do not possess. If you do, Proton is not the product for you, but it doesn’t matter because you can build and maintain what you need.