Max-Zorin So if my ISP / Network Provider can still see the domains I visit (and hence retain and sell that data) even if I use something like Quad9 as a private DNS, then is there even a point in using one of these services (besides say add/tracker blocking, family friendly filters, malware site blocking etc.)?
You are correct in that private DNS itself gives you little privacy since you are generally requesting DNS lookups to visit a site, but I would say there's some benefit to using a private DNS even if your ISP still sees where you go visit. They do use an encrypted channel to communicate between you and the DNS service you choose so they get around any tampering or filtering your ISP may intend to do regarding DNS although you could still be susceptible to them filtering or blocking requests to places you try to visit. Sometimes they are better than a ISP just because they may return requests faster or even at all, I have seen some ISP DNS just suck, possibly just due to updating their records infrequently.
Tampering with websites is something ISPs generally can't do thanks to the fact most communication is encrypted now and thanks to that encryption they can't see the specific URL you are visiting and only the domain.