That's not going to affect anything at 1G rates.

What is that port linked at on the modem? It's a 10Gbase-T port?

As I explained I would expect to find no way to set that adapter to 1G. It will only link correctly at 10G. That fact it works at all though seems to imply it probably is linked at 10G....

Yes, I would definitely recommend that. If only because that's how virtually all networks with VLANs are setup and if you do something unusual like that you will hit unusual problems!
Really the only reason to trunk tagged VLANs to a host is so that host can access multiple VLANs. So you might do that for a VM server or an access point with multiple SSIDs.

Steve

So you plan to have two connections between the modem and the existing router and you want to put the SG-2100 in one of them?

I thing we might need a diagram here.

Steve

Yes it will return anything that's defined locally directly. So host overrides and dhcp leases if you have that enabled.

Steve