Quad9 is decent, but there's some weird legislative issues (they can be court ordered to not resolve certain sites) BC weird reasons.
If you have a raspberry pi or similar sitting somewhere, you can set up a pihole DNS with unbound as upstream. Then you've got a DNS that's as private as you want, locally cached and with additional ad/malware/... blocking capabilities.
That may not always be the best way to go, as it'll make fingerprinting also much easier. The more custom your setup is, the less there are like you, the easier your tracked by fingerprinting techniques.
Not saying it's bad per se, but the idea that trusting no one and setting everything up yourself is always more private isn't true either. Both providers and do-it-yourself have negative sides one should stay critical about.
Okay, maybe I got the question wrong. If you care about content blocking, then you are right (though I'd prefer self-hosted resolvers like pi-hole or AdGuard Home over third party resolvers).
As far as I read (I'm no expert!) they could check the SNI of the TLS handshake if they want.
But using the DNS of the ISP is handing them the data right in a way they can analyze/use them very easily afaik?
They route your traffic, hence they can see all IP addresses you communicate with. With a reverse lookup you can then usually find out the address too.
I'm not an expert on what makes a "good DNS", but I have been using a pi-hole for about 5 years and it has been super stable the whole time, despite my best efforts.