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)
Thunderbird doesn’t have your private key to decrypt your Proton emails. The key lives in your browser and in theory there’s no way to securely provide that key to Thunderbird so it can do the decrypting. There’s a special application they built for business owners who want this functionality,
but by nature it breaks Proton’s security because the email content is then stored in plaintext (or close enough) so it’s not “secure” in the same sense Proton webmail is. (edit: maybe it got updates since I last looked, because the Bridge is now as secure as the webmail)You have to use the Proton Mail Bridge