I did try the internal temperature sensor, but it didn't measure the ambient temperature. I have read that if you set the ATtiny85 to sleep mode for some time, it will converge to ambient temperature, but it would take too long to validate, and I didn't want to go down another rabbit hole.

USB Bootloader is interesting, but I already had a few ISP loaders. You can make an ISP loader with Arduino as well. [1]

[1] https://docs.arduino.cc/built-in-examples/arduino-isp/Arduin...