Don’t use a thumb drive, use an external hard/solid state drive or install an internal drive. Even an aliexpress 64gb ssd for $10 is better than any thumbdrive. Thumbdrive’s flash and controllers are not designed for OS level continuous writes and will die very quickly.

If you must use a thumb drive, add some kind of air flow over it, and disable all logging features in openWRT to reduce writes as much as possible.