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)
Mailbox.org is a good pick to consider IMO. You can read some comparisons on PrivacyGuides, which I also recommend as a starting point for these sorts of topics. The mailbox.org web UI is not great, but it allows IMAP/SMTP access, so I use Thunderbird on both desktop and Android in order to interact with my inbox. My inbox is auto-encrypted with PGP using their Mailbox Guard thing, so my emails are all encrypted garbage on the web UI anyway. Mailbox.org only allows paid-for accounts, but considering the annoying stuff that Proton and Tuta do to their free accounts I’d rather just be honest about the service I’m getting. It allows auto-forwarding directly in the web UI, but given that you can hook up to it with IMAP anyway, it’s not like you couldn’t just do it yourself.
(Also, as another comment said I also recommend DuckDuckGo’s Email Protection for email aliasing if you need it.)