Force a single device to connect to 2.4Ghz or 5Ghz (could be possibly managed via DHCP client list or via address reservation list).

There are still many devices on the market which are not able to switch Wi-Fi band without the loss of connection.

Or there are occasions when router doesn't steer the client to different Wi-Fi band, even though the client has very poor Tx/Rx conditions (although Smart Connect is enabled).

For example, I have few Sonos One speakers at home, but only one of them is far from AX6000 router (having poor Rx/Tx conditions), but the router still keeps the speaker connected to 5GHz. Due to that, the speaker is completely unusable, because playback is continuously interrupting due to bad 5GHz signal.

On demand forcing of client to specific band would be a good solution for this type of issue.