Ty,
The wireless bridge is your 3rd subnet. Configure a seperate vlan on each of the switches (example VLAN 999) and assign and IP address to each side with a /30. Enable IP routing and add static routes.
On 35xx (vlan 999): IP = 192.168.1.1/30
on 54xx (vlan 999): IP = 192.168.1.2/30
On 35xx: ip route 0.0.0.0/0 192.168.1.2
which routes all traffic that is not local to the main site.
On 54xx: ip route 172.21.0.0/16 192.168.1.1
That assumes that the wireless bridges ethernet port are connected to VLAN 999 on each side.
Good luck,
Olaf