I just noticed you configured separate SSIDs for each band. Why not use the same SSID & Password for both bands and simply allow the router and wireless devices to automatically / dynamically select the most appropriate band? Ime with the 68U (also my primary router atm), this setup works very well: all of our wifi devices roam seamlessly between the two bands, depending on their current distance, radio interference, etc.
Against what seems to be conventional wisdom in this forum, I have not disabled any of the advanced wireless features: Airtime Fairness, Universal & Explicit Beamforming, AMPDU Aggregation, ACK Suppression, TX Bursting... even Bluetooth Coexistance: all of these features are enabled for both 2.4 and 5GHz bands on my AC1900P (AC68U). On our wireless network we have smartphones, smart speakers, tablets, laptops, desktops, gaming consoles, Chromecast devices, AppleTVs, and more ... all devices connect to the router using the same SSID/Password without any issues. WiFi channels too, Auto on both bands. I used to set the Control Channels manually (after running WiFi Explorer to monitor and asses co-channel & overlapping interference) but now I simply allow the router to do its job of assessing ever-changing airtime conditions and dynamically auto-select the best channels to use on each band, as it was designed to do. Obviously there are specific situations where channels need to be manually configured, but the 68U seems to do a rather fine job of finding the least crowded channels to use, at any given moment, without any wifi clients dropping off our network.
i should also mention that Asuswrt-Merlin version 380.70 is running on my AC1900P (68U). This 'Legacy' firmware may be out of date, but it's stable & reliable on the 68U.
I also have an AC86U in WAP mode (Asuswrt-Merlin v384.19) at the other end of the house, configured with the same SSID/Password as the primary AC68U router, and all advanced wireless features are enabled (except for Roaming Assistant, leave that disabled). I even set Protected Management Frames to Capable (enabled) on both routers, without any issues (Required management frames isn't recommended, unless all of your devices support 802.11w MFP).