I think that you may have still misconceptions about how 802.11 association works. It is a client decision, not an AP decision. Whether "mesh" or "normal" association, the client decides which AP to try to associate with of the options it has. The client decides if and when it leaves an existing association to move to another association. In general, clients are "sticky" and won't leave an existing association, even if a "much stronger" one exists. It is not at all surprising that your test client kept its existing association. As associations are break-before-make, there is no guarantee, from the client's perspective, that it can complete the transition, even with 802.11r enabled on the APs and a client that supports it.
It might be possible to create some "hack" that tried to measure the signal strength of all the potential clients across all the APs, share that information to a central controller, and forcefully deassociate the client from all but the AP that this complex process determined. The APs can't accurately measure the signal strength of the client due to clients using power control and that the client is only associated with one AP at a time. Even with that, the client could try to re-associate with the ones that keep disconnecting it as the client believes that they are the "best" option, from its perspective. If this were a significant problem that needed to be resolved, I would think that there would be commercial products and changes to the 802.11 protocols developed in the twenty years of 802.11 deployment. To date, I have seen nothing that manages the associations of non-cooperating clients across APs that seems in line with your goals.