We already have a thread discussing WiFi 6 mini PCIe cards. But putting them inside your mini PC isn't a good idea, these mini PCIe cards are meant to go into something with much more room for dissipating heat and routing antenna cables.
And often the best place to put your router is not the best place to put the wireless AP. I strongly suggest following @slh's advice and use the DL-WRX36 you already have as the AP. As a bonus, the DL-WRX36 can double as a 5-port managed hardware switch.
No, the 4 ports on the mini PC doesn't count, those are 4 separate NICs attached to the CPU. Any bridging between those ports require CPU involvement for each Ethernet frame. The DL-WRX36's ports connect to a switch ASIC that bridges the ports at line speed without requiring CPU resources.