Self-hosting an authoritative dns server is not a good idea for several reasons.
Cloudflare doesn’t have access to data hosted in your server unless you’re using their reverse proxy tunnel.
A momentary (if severe) blip in their availability isn’t a good reason to change providers.
Cloudflare is a business service primarily and the people who should be worried about its monopoly are the businesses, not so much a handful of people running home servers.