Correct, IP address 192.168.100.1 would be the default gateway for clients on the 100.0 subnet and 192.168.200.1 for clients on the 200.0 subnet.
You can also add devices from the 200.0/24 subnet to the DHCP Reservation list.
Are there any known drawbacks in using this function? Just know that this traffic is routed between networks on the same physical interface and it is managed with internal firewall rules.
The VLAN would be tagged, however the LAN port used for the 200.0 subnet would be configured as “access” so VLAN tagging is not required when plugged into that port.