Based on your config:
Set the 2.4GHz radio bandwidth to 20MHz (and the 5GHz radio to 40MHz).
Not related to the problem: you might want to increase your DHCP lease time, 10 min. is very low.
Were the external antennas of the hAP ax3 attached before taking it into use? Think the radio could get defected when powering it without the antennas installed. Based on information from the forum (I only know the hAP ax2 first hand) there is absolutely no discussion on which device should give you better Wifi performance.
In addition, you might want to consider start working with VLAN’s, instead of having 2 bridges. The latter does work, but you loose some hardware offloading. If you want to know more, read this topic:
http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1