In iOS 15 you can now customize text size per app. Make sure you first have the text size tool added to the Control Center.
To control the text size per app
Open the Text Size selector in Control Center, and you'll see an option to apply the text size adjustment to the system, or just to the app that's currently open.
iOS 15 will also remember your selection, so you can close out of the app to do something else and then return to it with your chosen text size for that particular app.