I actually found instructions from Netgate on how to do this from one of their web presentations.
Under firewall, NAT do a port forward rule:
Interface: OpenVPN
Protocol: TCP/UDP
Destination: Invert Match checked, This Firewall (self)
Destination Port Range: DNS (will be port 53)
Redirect Target IP: 127.0.0.1
Redirect Target Port: DNS (will be port 53)
This worked perfect for me and all Openvpn DNS requests are now encrypted with DOT. I actually duplicated this rule for all my interfaces/networks in case users try to use their own DNS servers over port 53, they will now be encrypted and sent over port 853 to cloudflare.