We found that 802.11r didn't help much, making sure that a device can see two other APs as it leaves the coverage of one worked better for us. It also helps with location tracking accuracy if you are interested in that. The other thing to be aware of is radio power levels, smaller devices often cannot raise their power level above ~17dBm so to ensure that the AP can hear them when they can hear the AP you should keep maximum power restricted to this or below.
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