Actually a A/E slot should only be x1 I believe, and it also carries USB 2.0 signaling, which some adapters can split off. x1 is plenty for a boot drive though, leaving the the M slots for other duties.Also remember that that 'wifi' slot is 'only' PCIe x2, so that's a max of something like 2Gbytes/sec if it's PCIe 3.0 (which I think it is). Some NVME SSDs are faster than that.
Honestly I’m pretty surprised no company has bothered to make a native A/E 2230 SSD yet.