Be sure to check OPNsense Ethernet card compatibility. When I built my router, it was strongly advised to stick with genuine Intel PCIe network cards. And I’d personally strongly recommend against using a USB to Ethernet adapter. To many reliability issues with the USB side of things.

In a non-conventional setup test, I tried an old Mac mini with two thunderbolt to Ethernet adapters and while OPNsense worked, stability wasn’t optimal.

Of course, your mileage may vary and genuine Intel PCIe cards are quite a bit more expensive than a USB Ethernet adapter.

Use your own router downstream of theirs.

Does your server has an empty pcie slot? If so I would go for an ethernet pcie card instead of usb-c adapter.

Or, more expensive, go for a qotom router and migrate opnsense to it.
f.e. something like this https://teklager.se/en/products/routers/tlsense-J6412-aesni-router

Come out of the ONT directly into your OPNsense install?

It’s what I do with my fiber install.

I use a TP Link ER605 and it works well. I had to ciall my ISP to get it enabled but that says easy.

I know you said you haven't got any PCI slots available, do you have any M.2 slots available? You might be able to get a M.2 to ethernet if so.

Stessa tua situazione: a valle del coso di Fastweb ho il mio server linux con due schede di rete: una è usb-c.