If you live in densely populated areas, I recommend flashing while in the basement/cellar or even going to a park or in the woods.
For me, flashing these thermometers with ATC firmware worked much, much better while in the cellar.
Personally, the only advantage I see for ZigBee is the relaying of the network (my Pi4 could be better hidden away). Otherwise, if the reports are every 5 minutes, irrelevant if the values change, it would make my bathroom fan automation almost useless.