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.
(source)
I mostly use my custom domain on protonmail for reasons like this.
I use Tuta but it’s what convinced me to buy a domain. So if/when I switch providers the majority of contects will not notice.