I'm interested in building a DIY router and would be interested in recommendations for various bits of hardware especially motherboard and case.
I would like to install a 4x1GB Intel NIC adapter and also a PCI-E adapter for a 4G/5G modem with a SIM card slot.
Is size and power consumption parameters to be considered?
As small as possible. A two port NIC would be sufficient if I found a really compact case and motherboard. It prefer it to be powered by an external 12v power supply.
For two ports, get a sw302da (or better) or the APU2 (or better), they come with at least 2 intel ethernet ports, and at least one SIM slot - https://pcengines.ch/apu2.htm
Roqos RC150 would check all of your boxes, even comes with a 4G modem, so no work required,
but it's bigger.
I was thinking about something like:-
I thought that APU2 motherboards were quite expensive.