I also agree on setting up a custom domain for your email. I did it myself a while back and I can’t believe I didn’t do this like 10 years ago.
Cheap domains on Namecheap.com are nominal. A .com domain right now is only $6, and make the hosting cost the only real consideration. Setting up the domain with hosting takes minutes, but you want a legit vendor doing this so you get SPF, DKIM, and DMARC scripts that legitimize your emails. That’s what you’re paying for, plus someone else dealing with spam, which self-hosting won’t do for you.
$300 a year for email hosting seems crazy high. Mailbox.org’s more modest options for 1 user are $4 a month with docs. Proton requires an Unlimited plan for custom domains, which is $10 a month - but you get a VPN and docs included, so if you’re paying for a VPN (and aren’t a heavy torrent user, IIRC, Proton doesn’t love that), that’s a cost you can consolidate. Tuta will let you add a custom domain to a free account for EUR 1 a month. So you can get this all set up for under $50 a year. Maybe even under $30 a year. You can’t get a burger and a beer for under $30.
In the grand scheme of things, paying for email hosting means you are not the product. It’s worth it and makes life measurably better.