So your problem is that your OpnSense has LAN on vmbr3 aka enp90s0 and it should act as a DHCP server (which it does according to your packet traces) but your clients attached to that NIC (via a switch?) do not get DHCP IPs assigned?
If the NIC is not attached directly to a client, but via a Unifi switch or the dream machine itself, you should know that there are settings in Unifi to block non-legit DHCP servers. Maybe your Dream Machine is the only allowed DHCP server. In order to verify, try to attach a client directly to that NIC (make sure to use the correct one).
Also, since your PVE host is already attached via some bonded SFP+ NICs, I would rather use those instead of another NIC. You can distribute VLANs over that NIC(s) to do that. Besides that, I am not a huge fan of bonding except for reliability. With Unifi equipment, you will gain no more throughput via bonding, because most Unifi hardware does not support "real" load-balancing. I think that balance-rr would not work in the way you think it does.
If the NIC is not attached directly to a client, but via a Unifi switch or the dream machine itself, you should know that there are settings in Unifi to block non-legit DHCP servers. Maybe your Dream Machine is the only allowed DHCP server. In order to verify, try to attach a client directly to that NIC (make sure to use the correct one).
Also, since your PVE host is already attached via some bonded SFP+ NICs, I would rather use those instead of another NIC. You can distribute VLANs over that NIC(s) to do that. Besides that, I am not a huge fan of bonding except for reliability. With Unifi equipment, you will gain no more throughput via bonding, because most Unifi hardware does not support "real" load-balancing. I think that balance-rr would not work in the way you think it does.
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