> The obvious advice for roaming is “use one SSID everywhere”, and that is often correct if you’re running Wi-Fi in an office, a public venue, or generally somewhere where you don’t have (or care about) legacy devices.

What difference does the presence of legacy devices make? Is the intent to isolate them from modern devices from a network perspective? Then create a separate SSID on both 2.4 and 5 GHz for modern devices.

I can't think of any legitimate reason for split SSIDs anymore. Linux clients used to be pretty bad at preferring 5 over 2.4 GHz if RSSIs were both excellent but 2.4 was slightly better, but I haven't seen that in years.