It's a losing battle trying to escape from the dreadful iOS photos app.
All apps that can access your photos default only to the iOS Photos app. If you try to store images in some other photo app, other apps won't be able to access them.
The camera saves to the iOS photos app too. No other choice. So you'd have to constantly copy stuff over from the iOS photos app to a third party app.
But then you'd forget if you copied everything over, so you'll inevitably try to copy photos over numerous times.
The iOS photos app is such a lame piece of software. No ability to tag or name or search your photos except by scrolling through endless squares. And the Albums feature sucks too.
So yeah, it's lame and we're stuck with it. And no, you can't change the default location for photo storage. Everything has to end up in the dumpster called iOS Photos app.