There are quite a few reasonably priced mini PCs with (typically) four 1000BASE-T network cards offered on amazon or aliexpress, for cheap used offers look at Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US.
The only real obstacle would be your requirement for two mini-PCIe slots, that is basically not on offer - unless you either go very expensive, or large (µATX); one of the few exceptions to this would be the roqos core rc10. On the other hand, using a classic 802.11ax 'plastic' router running OpenWrt to outsource the AP functionality is both (considerably-) cheaper and usually better performance (better antennas, better placement, away from the 'big' metal case of your PC and its interference).