Pi-hole works great for me, but everyone else on the network that uses Google hates it because the entire first page is ads and they can’t click on them.

I second your idea of going with Pi-Hole. It is purpose-made for this and easy to setup.

I use Adguard’s public DNS on my router for convenience, no problems at all. In the past I had pi-hole with some lists that in the end, from time to time, broke things.

If you want cheap and easy, something like NextDNS. Otherwise, your tentative plan works just as well. My family liked NextDNS because all I had to do was have them install an app, enter my code (for the profile I configure for them), and set it to on. The rest was magic, to them.

Put their router into modem mode and daisy chain your own router. Look up its labels and find the original manufacture for manuals. Watch out for name changes and mergers. ISPs do not manufacture routers. They buy them from companies, change a few logos and lend them to you.

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NextDNS or many other DoH services that are out there (I personally recommend Mullvad).