An m.2 NVME SSD in a PCI Express adapter will work an will be faster than a SATA SSD, but will not be bootable in a motherboard whose BIOS/UEFI does not have native UEFI support (there are workarounds/kludges around this) . As mentioned by mihai, although there is a performance difference, which can appear quite significant when measured in a synthetic benchmark, in practice, for something like gaming it is unlikely to be very perceptible (i.e. 4 times faster in a synthetic benchmark, for example, may seem significant, but when that translates to something like 12 seconds versus 3 seconds or even less of a difference when CPU processing time comes into play, I don't personally don't see a reason to care).
As for future proofing, I doubt SATA is going away completely any time soon on desktop motherboards and even if that is a concern, PCIE SATA controllers are common and cheap .
Future proofing is a nice concept, but the way you mention it, the planned upgrade point is in an undefined future, I would not spending much time and effort planning for it as
a) SSD drives will get bigger and faster over time
b) SSD drives will get cheaper over time (at least in the long term, after that crypto goldrush hopefully blows over)
In that context, spending more money now to get something better than what you need or can actually take advantage of in the hope of taking advantage of the extra functionality/speed later on when better and cheaper alternatives will likely be available does not seem worth it to me . Additionally, seeing that you are posting this on Vogons, one would assume you that have an interest in things retro and likely still will down the road, so reusing an "old" SATA SSD in a gainful way should not be a problem.
Finally, unless, you actually need/want 2TB or more of game storage now (considering that you are currently living on 2x100GB, I doubt that you would need 10 times more right now, correct me if I am wrong), getting an SSD that big is not worth it right now, IMHO. For example, in my neck of the woods, looking at Samsung 870 EVO series SATA SSD prices (QVO have less endurance and are thus even cheaper, I personally wouldn't go there, maybe you would, we all have different comfort levels/needs on that front, other manufacturer) as an example :
(prices are in Canadian dollars from a competitive local retailer)
Samsung 870 EVO 250GB SATA III Solid State Drive : $57.99 --> 0.23.196 dollar/GB
Samsung 870 EVO 500GB SATA III Solid State Drive: $89.99 --> 0.17998 dollar/GB
Samsung 870 EVO 1TB SATA III Solid State Drive : $169.99 --> 0.1699 dollar/GB
Samsung 870 EVO 2TB SATA III Solid State Drive : $269.99 (currently on special from 324.99 )--> 0.134995 dollar/GB or 0.162495 dollar/GB at regular price
Samsung 870 EVO 4TB SATA III Solid State Drive : $624.99 --> 0.1562475 dollar/GB
Of course, the current special on the 2TB drive makes it a more an interesting proposition currently, but otherwise, you start getting pretty decent value from 500GB onwards . Whether you need 2TB now is something only you can know or maybe guess at and, to be clear, I am not advocating that you get the smallest drive you can (planning for growth does make sense). My point is just that too much future proofing and capacity over-planning can cost more money than it saves.
Interesting graph of price history and projections in this article :
That's just my long-winded 2 cents of an opinion .
Happy planning/shopping .