Client balancing can be enabled under the RF profile. It appears you do have that enabled. Looking at your event log it shows client balancing is being used as expected. You can see this under event type "802.11 association rejected for client balancing ".
Band Steering is also enabled on your network. That persuades dual band clients to join the 5GHz network. This is typically good to have enabled unless you see a specific scenario in which it's causing an issue.
Clients have their own programmed routines for when/how they roam. Apple has this documented for iOS devices. So, just because your device might be physically closer to a certain AP doesn't automatically mean the device would or should connect to it.
So with all that said it mostly comes down to the client to decide how and what to connect to.
Also, I see 10 of the 15 APs have their 2.4GHz and/or the 5GHz channel statically configured. Is there a specific reason that was done?
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