So as a rule of thumb, sqm works pretty well on fixed rate links. On variable rate links the best we came up with are IMHO the different autorate projects like cake-autorate and lua sqm-autorate and perl sqm-autorate. That however requires active RTT or better one way delay (OWD) measurements, that are relatively easy to perform against servers in the internet. In theory this would also work for WiFi but you would need to run an independent controller per station, and IMHO that is not a realistic option.
So ATM the pragmatic way forward IMHO is to pick an AP that implement airtime fairness as well as trying to connect all truly latency sensitive devices via ethernet cables directly to the router...
But I understand, that is not always possible and so compromises need to be made, or rather local policy needs to be set, and I think each network admin needs to find the best policy for their own network.