Your boundaries on cost make it tough, but aside from Tuta you might have a look at mail.ee which has very basic features (no E2EE for example) and a retro web UI, but very high storage limits. They offer free accounts too, and support SMTP/IMAP/POP3. It’s Latvian-based so comes with the “100% GDPR compliance” feature if that’s of interest.

Zoho.com is another that comes to mind. It’s very feature-heavy/slick (you can tell they’re attempting to market mainly to small businesses looking for a cheaper Google Workspace), has been around a long time and I’ve read positive comments from others about the service. It’s an Indian company though so you don’t get GDPR protections (or similar) as far as I know. The low-end plans are in your price range and I think they still offer a free plan - that’s what I have anyway.

I’ve been a Fastmail customer for decades now and it’s exactly what I want a mail service to be, but it’s out of your price range and has no free tier.

skiff.com might be worth a look. Its services are E2EE. Its a lot like Proton in spirit but with better pricing and less nickle-and-diming. 10G of storage on the free plan. It’s not a Swiss company though, if that should happen to be important to you.