Am in the same position, own domain which I use DNS challenge with cloudflare to issue certs to local systems and which auto renew - e.g. Proxmox, Pihole. I do this to remove the annoying nags from Browsers about invalid certificates which are getting more and more fussy. My systems are not exposed to the internet (I run tailscale for remote access)
Feature request would be for QNAP to add full ACME functionality to their Certificate Control Panel. Paying for Certs on someone else’s domain in 2026 isn’t a great look.