With sensor deployments as large as what you have described, there indeed needs to be confidence in the network, particularly when there are outages.
Surely there is a threshold which The Things Network considers “acceptable” for the frame counter being above that expected. By knowing this number, you could be able to determine how long an outage would need to be in order to require a rejoin.
I appreciate that experimenting may lead to an answer, but isn’t there a max frame count gap programmed into the configuration to replace the previously defined standard of 16384?