I’m honestly a bit surprised that Proton doesn’t seem to have the send as feature. I was able to find at least 15 posts across their uservoice.com site and their Reddit forum, spanning at least 6 years, with one of the uservoice posts having over 300 votes. I just gathered up all the links and sent it into Proton Mail support. Hopefully having all that thrown at them in one big bundle will prompt their project managers to consider it.