Personally, I have NEVER used "Smart Connect" and never will. I don't want the router mucking with the connections period. As a best-practice, I create a different SSID for 2.4/5 GHz band on each router. I can look at the name of the SSID and know exactly what I'm getting or if there's a problem, then I know which routers is down. Maybe I've been lucky but in the past 5+ years, we never, ever get disconnects unless I'm mucking with the routers in upgrades or AMTM stuff. This is on both RT-AC86U/AC88U/1900U models. I think heading down that same road may help stabilize your wireless setups. Cheers.
Yes, I do what you do and it works well.
Furthermore, I don't add both bands to my devices; each device will only get EITHER 2.4 or 5.0, not both. This way I don't have to deal with a device flipping back and forth between 2.4Ghz and 5GHz.
Typically I have my IoT devices and security cameras on 2.4 and more modern smart devices (phone tablet laptop chromecast) on 5.0. Theoretically when I go on vacation I can turn off my 5ghz network and leave my 2.4ghz network enabled, to reduce a potential attack vector.