What’s the resource usage like? I’ve looked into mailcow before but the recommended system specs are too expensive to make it worth it on a VPS.

Have you informed the elders of the internet about this?

I use Technitium, for that purpose. You can set up DNS records easily and it still has blocking like pi-hole. You can log DNS requests if you need to track down where certain requests are coming from or which devices are making lots of requests. It has quite a few features but I only need a couple.