Both auto-forwarding and auto-reply are paid features, which makes cancelling & switching much more difficult. Gmail is a breeze comparatively. I highly recommend against using their addresses (e.g. protonmail.com, proton.me, pm.me)
Email forwarding is available for everyone with a paid Proton Mail plan.
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To get notifications using the Android app on a de-googled phone, you can use the app ‘You Have Mail’ available on F-Droid. It was developed to solve the issue you described.