Have a subdomain whoose only purpose is Cloudflare redirecting before it even hits the origin?

Change the DNS record to a AAAA record that points to 100:: and, obviously, make sure it’s CF Proxied (orange cloud!). It now points to nothing but Cloudflare Redirect rules will still work. Cloudflare will automatically make a corresponding A record for IPv4.

Cheers Lewis. Apparently 100:: is the official recommended IP for this purpose.