I don't believe it's possible with non-Google Domain hosted domain names but I own a number of domains that I use to send and receive from my root @gmail.com account for free (Google is my registrar for these). They come in to @gmail.com from @mydomain.com and when I reply it goes out as @mydomain.com. I can also send email from any of my @mydomain.com emails by opening compose and clicking an arrow on my from address.

There is the huge possibility that this will cause issues with some mail servers because I'm assuming it's doing a send on behalf of (I could be totally wrong). But so far (1yr+) I don't recall any issues.

I switched to Proton for a bit but I just didn't enjoy using it at all. The final nail in that coffin; I was in the middle of selling a house and receiving documents and tons of back and forth and it was an absolutely terrible experience. Every realtor/contractor/etcs signature pictures, etc showed up as attachments and the search was awful so having to dig up contracts and everything else that goes along with a house sale was just an absolutely miserable experience. Every single email looked like it had tons of attachments. I had to go through every single email in a thread to find the actual documents.

So I went back to gmail.