FYI, someone submitted a firmware upgrade image file for Bosch BTH-RA (a.k.a. Smart Thermostat 2) to https://github.com/Koenkk/zigbee-OTA/ as per this PR https://github.com/Koenkk/zigbee-OTA/pull/279 → https://github.com/Koenkk/zigbee-OTA/tree/master/images/Bosch
If you are using the ZHA integration then a tip is to use ZHA-Toolkit to download and upgrade that Zigbee OTA firmware image on that specific device once you have successfully paired/joined it to ZHA → https://github.com/mdeweerd/zha-toolkit/blob/main/README.md#ota_notify—downloadtrigger-device-fw-update