Agreed. Emails received from a simple login forwarding alias should default to the simple login alias in the 'Sent From' field if I reply to the email.

Maybe I'm misunderstanding the question, but isn't done using a reverse-alias?

Specifically, under "Aliases" in SimpleLogin, find the alias you want to send as then click "Contacts", enter the email you are sending to in the textbox, "Create reverse-alias" then copy that reverse-alias to use as the recipient in Proton Mail.

Agreed it's rather convoluted though (it's simpler if you are replying to an email, as SimpleLogin automatically replaces the actual senders with these reverse-aliases), and it would be great if Proton Mail had more seamless integration with SimpleLogin (e.g. select to send as alias which would then auto-create/substitute the reverse-alias).

Strictly speaking, there is a way to do this, but it is a bit of a pain in the neck.

You can reply to an e-mail that came to an alias and it uses that address in the reply.

So send yourself an e-mail to that alias. Then whenever you want to send an e-mail using that aliases as the sender, reply to the mail you sent yourself and change the title and recipients name and get rid of the text in the message you sent yourself.

Try it. Suppose your address is example@proton.me. Send yourself an e-mail to example+3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067982148086513282306647093844609550582231725359408128481117450284102701938521105559644622948954930381964428810975665933446128475@proton.me. Then use reply to send a message to a friend.

Note that one downside to using aliases is that the PGP encryption no longer works. Each '+' alias you use would need its own PGP key. If you could identify '+' addresses that you would like to use for encrypted e-mail, it would be nice to be able to select those '+' addresses and create a PGP key for each of them.