If you mean a DNS sinkhole, like Pi-Hole, then that shouldn’t be a problem. Pi-Hole acts as a DNS provider, so:

• It could also act as a DoH provider. Someone seems to have done this for Pi-Hole here: https://discourse.pi-hole.net/t/can-pihole-act-as-a-doh-or-dot-server/56106
• You could disable (or not enable) DoH in Firefox to make regular DNS requests at your Pi-Hole, but then your Pi-Hole can use DoH to forward the DNS requests to outside DoH providers. Pi-Hole seems to support with either cloudflared or dnscrypt-proxy.

On Android, there’s also ad blocking solutions that run a VPN server on your device, to terminate certain requests that way. I’m not entirely sure how those work, but if needed, they could presumably also run a DoH provider on your device.

Only if your adblocker doesn’t support it. Pinhole works just fine.