It really depends on how you have your /etc/network/interfaces set up. For one of your bridges, proxmox needs to have an IP. If you want proxmox’s traffic to go through OPNsense, it should have an IP on the LAN bridge. You have to make sure the interfaces file explicitly sets a static IP or explicitly says it will get its IP via DHCP.
Since you set a static IP on OPNsense for Proxmox, you will need to manually set it to use DHCP on the LAN bridge. In my experience, this does not work because Proxmox will fail to get an IP via DHCP if OPNsense is not up yet. I highly recommend you set a static IP in the interfaces file.