Mmmh, sqm requires you to set fix rate limits and will only work well, if these limits are below the actual wifi link rate it can work (and considerably so, for wifi I would guess you need to set the shaper to 60-70% of the reported stable link rate). But since wifi link rates depend on your RF environment and distance to the router, they typically are not stable but vary over time, making the sqm approach somewhat sub-optimal for wifi...
For the ath9k and more recently for the ath10k atheros/qualcomm wifi driver there is an airtime fairness mode implemented which does fq_codel in the wifi driver itself (which nows the link rate and hence can always work with a decent and recet estimate of the available linkrate) which helps quite a lot. I was under the impression that this would work for your mediatek based radio as well. I wonder why you see issues at all.