You can buy a PCIe riser for your NIC. This could be a solution for the GPU taking a lot of space. Naturally you still need a slot on the case to attach the NIC to, but at least you will be able to put it into a PCIe slot.
Look for chipsets that advertise Linux Kernel 2.x support those work right out of the box typically.
Stuff people recommend for the raspberry pi should also work fine.
Try for USB-C, if you can't find anything good there. Just make sure you're not getting a USB2 device or 10/100 nic.
I'm using two of those which come with the RTL8153 chipset. They run fine on my Proxmox server. No seperate driver installation needed.
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