For the main building I've placed a firewall just before the router
Based on your drawing, you've placed the firewall after the router, in other words the firewall does not protect the router itself but rather sits downstream from it.
There's not necessarily anything wrong with that, except it means that the uplink from Building 1 to your main building will not be protected by the existing firewall if it is connected to the router in the main building.
Using two firewalls for your situation would introduce unnecessary complexity, cost, and management in my opinion. Instead, think about connecting Building 1 to the existing building by either interconnecting either the two routers together with the firewall placed outside of the router in Building 2 or alternately, connecting the two "main" switches together. In either of those scenarios, both buildings' networks will be protected from the internet by the existing firewall.