As an alternative opinion, I have found that roaming is best with using a separate SSID for each frequency. I optimize the positioning of my APs based solely off 5ghz to get that light overlap at about -65dBm between the APs to encourage a good handover both via router assistance and via client preference. I’ve found it is important to pay attention to both the physics / hardware side of things and to use assistive technologies from the software side.
2.4ghz and 5ghz have different range and overlap differently - so roaming depending on both can be a big mess. It introduces the possibility of multiple jumps (both between bands and between APs). I’d rather jump once (AP => AP) instead of introducing the possibility of multiple jumps including to slower / more congested 2.4ghz band (worst case: AP1 5ghz => AP1 2.4ghz => AP2 2.4ghz => AP2 5ghz). Each AP utilizes a non-overlapping channel for both 2.4ghz and 5ghz.
Ultimately the client will always choose when to roam. I have all apple devices and they are exclusively on the 5ghz band (IoT devices are exclusively on 2.4ghz). I’ve gone so far as to name the SSIDs “Fastwifi” (5ghz) and “slowwifi” (2.4ghz, lol no family member / guest has asked for the slowwifi password). Everything that can be hardwired, is hardwired. I’ve had really good roaming and happy family members with this simple split SSID approach.