With the help of DNSFilter support, I was able to figure out how to set up multiple VLANs to use the DNS relay.

Immediately below the local DNS servers section of the relay.conf file, insert a LAN subnet section with your subnets specified in CIDR format.

Example:

[[lan_subnets]]
addresses = [ "10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24", "10.0.4.0/24", "10.0.5.0/24", "10.0.6.0/24", "10.0.7.0/24", "10.0.8.0/24", "10.0.9.0/24", "10.0.10.0/24", "10.0.20.0/24” ]

Make sure you go to the DNSFilter website under Deployments > Relays > LAN Subnets and specify your subnets and their policies.

Works great!  Unfortunately, I spent a lot of time troubleshooting this because I didn't have a return rule from my Synology NAS hosting the relay to return the result to clients on different subnets.  Once I realized and fixed that, it's been working great.  Next up is to set up a second relay on a Raspberry Pi to serve as a backup.