> It does need client-side support though.

Not really! You can buy a router that ships with OpenWrt out-of-the-box and just toggle a little checkbox. Plug that into your ISP's router (or use a wireless bridge in client mode, that's supported, too) and connect all of your devices through that. Now all your devices use DoH and don't even know it.

No need for client support, you could just deploy it on a Linux vm running somewhere on your network and let that be the dns server served via dhcp.

For extra points you could deploy a firewall which intercepts all DNS requests and forwards them to that machine. Some apps have hardcoded DNS servers and ignore what you have configured.

ISP can simply compile a blacklist of publicly available encrypted DNS resolvers and block them.

not really feasible for non technical folks but at that point you start to run a dns proxy in cloud with static ip and proxy all your dns requests using DoH to that IP. That would be really hard to block without blocking all outbound https connections