OK, I did some additional tests with iOS and Windows 10. Definitively changing ft_over_ds='0' and reassociation_deadline='20000' did improve roaming for the Windows 10 laptop as recommended in the bug report you referred previously.
While I am still getting some lost packets while moving from one AP to another, roaming with Windows 10 is now better/faster than without 802.11r. So at this moment I will keep 802.11r enabled.
However with iOS roaming is not much better with 802.11r. While at some moments I was able to roam with no packet loss on the iPad, it was just on one or two occasions. I believe this is related to the error "hostapd: nl80211: kernel reports: key addition failed" errors which seems specific to iOS. As mentioned in the bug report, this may be the result from iOS keeping open connections to multiple access points and this is causing FT roaming to not work properly with iOS devices when moving back to an AP previously authenticated.
Well, since I'm the OP, I will mark this post as a solution to help anyone with this issue in the future to quickly find this information. I'm also adding below the reference to bug report #3159 that did shed some light on this issue. Once again thank you @craz for finding and sharing this bug report! 