https://www.rainloop.net/ is actually much better software in my opinion, and is self-hosted unlike FastMail.
Rainloop satisfies the need for polished, smooth, elegant, yet simple "feeling" interfaces like Gmail finally became.
Handles multiple identities, multiple accounts, and contacts well; but no calendar.
Way better than Zimbra webmail, though that does have Calendars. On my local machine, I still use RainLoop rather than a traditional email client, it's that good.
ProtonMail, also mentioned, doesn't seem far off, but seems to lack the extra polish. It's in the neighborhood of RainLoop though, it would appear.
Firefox, DDG, Fastmail, etc are all perfectly good enough and don't make me feel dirty.
These days I host my own anyway, and have a blargh@host mail account I use to sign up for things (typically blargh+thewebstie@host so I can track who is selling email addresses)
There's also the option of running your own server, but it seems deliverability is an issue.
Move to Fastmail if that's what your after. You are using a free service, because you are the product. That's not about respect, it's about your (lack) of understanding of the business relationship you entered. If you aren't comfortable with that, you're going to have to pay actual money.
The reality is that irresponsibility and disrespect for users are rampant in the tech world. That’s entirely companies’ doing.
If you use a free service, you basically have assurances that they're not respecting your privacy. The probability of a paid service respecting your privacy is greater than zero, at least.
Also, not all paid services are created equal. You still have to do more due diligence past just throwing money at someone who'll take it.