Can't you just put a real PCIe NIC in an eGPU enclosure with a fan? I bet you can solve this problem for like $150 and two craigslist transactions.
If you are unfamiliar, the reality of the situation is quite bad. Slight hardware revisions of adapters/switches, which if you are lucky can be differentiated by manufacture date or serial number sequence, will not recognize known good modules, and if they do you then have to actually test if they can establish link, then let them burn for 24+ hours to see if they can maintain it without dropping (often, because of forward error correction, you will not start to see flapping until enough correctable errors have accumulated and the buffer overflows). Then there are firmware updates where this process must then be repeated.
You would think with how expensive all of this shit is you would simply be able to buy the officially listed as compatible components but my experience with them has been that a shotgun approach is best for your sanity and deadlines, buying sample kits of each module and trying them all, and then once you find the ones that play nice, buying as many of them as is reasonable and testing all of them.
If I had a dollar every time two modules from the same brand, that came in the same box, from the same production run, did not work with one another, I would be able to test your theory for free. Network Engineering is Fun