Looks like 19.07 is still missing the latest master commit to expose the hearing map “hostapd: expose beacon reports through ubus”. r works in 19.07- Going to need to update to master to experiment with k & v.
Haven't tried updating to master mainly because of having to do extra work to get luci back plus it probably is inherently not as stable (and also I have never done it before although I do have a spare raspberry pi lying around if that's what it takes to get a master ready as my router is not yet supported in this whole ar71xx transitioning to ath79 process etc etc).
But I now have a workaround solution that's almost as good as having 802.11 k and v. So like you said r is already working in 19.07. I have that enabled, then use wifi-disconnect-low-signal that I got from github which allows my router to kick someone out when the signal is too low. Turns out after tweaking some of the parameters this works really well. My only minor gripe is that when both of my routers provide adequate signal, the device would not know if it is connected to the stronger of the two - which I suspect would be case for a full fledged 802.11k/v setup. But at least they are now roaming when they should to maintain connectivity.