Annoyingly, I can't really do 2.5G because when I was looking to upgrade my network, I ordered a bunch of 15 year old old 10GbE switch, because I mistakenly assumed that that would work with 2.5G as well. It turns out that the 2.5GbE came after 10GbE and so compatibility is not guaranteed, so I ended up having to upgrade everything in my house to 10GbE to make the switches work. Fortunately, used 10GbE cards from data centers go for around $40 on eBay. Thunderbolt cards cost a lot more so I only have one for my MacBook, and it's a big chunky boy that weights half a pound.

I was looking into some of those single-purpose computers, some of which will sometimes have a PCIe slot.