Yup that’s it, you were quicker than me, thanks!

It’s somewhere in the settings on the web interface, you can set up SMTP credentials for any of the email addresses you’ve set up in Proton there. Maybe you’ll find it on your own already, but I could look it up exactly once I’m on the PC later

That’s wrong, you can set up sending via SMTP pretty easily. And they also have an IMAP bridge app to access your mailbox through other clients like Thunderbird.