Comment by guerrilla

I think your best bet might be NextCloud. You can either host it yourself on a storage VPS or find a provider who provides managed NextCloud (or however they repackage it.) Be aware there are many more providers than listed on the NextCloud website and you'd do well to search about. If you're in Europe, take a look at Hetzner's "Storage Share" as an example of what's possible and prices for high-quality hosting, but like I said there's a million operators out there doing this and you can get it dirt cheap if you want.

I don't know how it works on an iPhone but on Android NextCloud detects when save pictures in a different directory due to some app and asks me if I want it to track that directory. It was already configured by default to track stuff taken with the default camera app.