Your OptiPlex will have considerable PCIE expansion though, so you could slot in a second hand dual-port nic if you wanted to (10GbE might be easier to find than 2.5 and they are still relatively inexpensive as second hand hardware)
Siege is honestly awful these days (not complaining about the sci-fi ops, reality is lame as shit why not spruce it up a bit) but what they have done to the UI and the queues (bring unranked back please) is honestly unforgivable and makes the game hard to play. (It isn’t all bad, but more bad than good)
On grapefruit, when was the last time you used it? There was a bug on roblox’s end a little but ago and it would make the client crash after a few minutes in game, but I believe it has been fixed (also vinegar is better imo :p)
They admittedly seem to have gone up in price since I last looked, but here is a listing for only $40