I am not so optimistic that install-code and qr-code support will be added to ConBee / RaspBee soon, so if you need/want to use this feature today then probably best to buy a new Silicon Labs or Texas Instruments based Zigbee Coordinator adapter instead, see → Zigbee buyer's guide
FYI, ChrisHae from Dresden Elektronik wrote a reply there that ConBee / RaspBee firmware deconz serial interface does not currently support joining/paring via install code (or QT code) but a firmware update that will add it is on their roadmap for ConBee 2 / RaspBee 2, (though not for the original ConBee / RaspBee which firmware is no longer getting updated features), see ChrisHae’s reply here → Does deCONZ serial protocol for ConBee/RaspBee Zigbee Coordinator adapters not support joining/pairing via install code and qr code? · Issue #20 · dresden-elektronik/deconz-serial-protocol · GitHub
One possible workaround could be to migrate your Zigbee network in ZHA to a newer/modern Silicon Labs EmberZNet (EZSP) or a Texas Instruments Z-Stack (ZNP) based Zigbee Coordinator adapter, (like, for example, a “Sonoff ZBDongle-E” or a “Sonoff ZBDongle-P”), as those support joining/pairing via install code and qr code, and then migrate back (or not), see → https://www.home-assistant.io/integrations/zha#to-migrate-to-a-new-zigbee-coordinator-radio-inside-zha
Another related tip specific to the Bosch Thermostat 2 device is that it might be worth alsy try upgrading the OTA firmware on that Bosch Radiator Thermostat II device (via Zigbee2MQTT), as that was at least mentioned in the Zigbee2MQTT community → [New device support]: Bosch Radiator Thermostat II · Issue #14926 · Koenkk/zigbee2mqtt · GitHub
By the way, for manually upgrading firmware in ZHA after a device has already been joined/paired, see ZHA Toolkit → https://github.com/mdeweerd/zha-toolkit#ota_notify