PrivateFirewall and Internet sharing (ICS)

I trying to enable internet sharing (ICS) between pc+wired router and wifi pci card. From my observations I gathered that the 'system services' firewall rules are blocking the connection between the wifi network (192.168.137.0) and lan (192.168.1.0). The way I found to allow ICS is to completely allow system services in the applications section or to allow all tcp requests (or other necessary connections) with a customized rule.

Are those methods open my computer to possible threats?
Is there a better way to solve this problem?

Thank you.