I do this. Its great because of catchall emails and the ability to make one address per merchant. Then if a company leaks your email or gets hacked, you can simply change the email from hacked-company@domain.tld to hacked-company2@domain.tld and block the old address.

It also is good for ownership as you said. If Tuta gets purchased by Google (for example), then you can simply pivot to any of the many other email providers and not rely on a company being not evil.

Google gets lots of your email either way, since many of your correspondents will be on gmail. I've been getting domains mostly from porkbun.com which offers free whois privacy. namesilo.com has it too.

Domain registrars are required to collect your real information and display it in the Whois database. Many registrars offer a "Whois privacy" service where they put their name and address in the database instead of yours, most charge a fee for this. That's one thing to keep in mind.

Your own domain is not great for privacy though like others have said the registrar can hide your info at least from whois. If you already have a domain lookup the whois record and see what it says. Presumably even with whois privacy your identity is probably discoverable.

Custom domains are not great for deliverability too. Though mostly mine is fine. Sometimes Yahoo and ATT manged accounts give me delivery issues.

What your own domain is good for is nice, long term, and portable addresses. Also for many cheap addresses. I get something like 30 email accounts with my basic Namecheap cPanel account for about $25 per year.

You could find a provider that lets you bring your own domain, and maintain that instead.