Update: The new Schlage FE599NX came yesterday afternoon and I excluded the old Kwikset 912 and added the Schlage to the hub, all apps, DTH and automations. The whole process went very well and, so far it hasn’t dropped from the Z-Wave mesh at all. I know it’s a relatively short time to judge, but by now, the 912 would have dropped a minimum of 6 times - probably many more.
First impressions of the Schlage is that it’s much quieter, responds much more quickly to commands and automations and is much more sturdy than the 912.
Good luck to those of you trying to solve your Kwikset connectivity issues.