Which build of firmware is proven to be stable?
You didn't specify which Drone, but for the Q500, Typhoon H and the H Plus, the current firmware is about the most stable available. SD card Downgrade or Upgrade comes into play only rarely. It is most likely needed if something went wrong with a normal OTA upgrade, or if some quirk in the system can be resolved by temporarily downgrading to allow a new OTA upgrade to work.
I also would like to upgrade an older cgo3 I have to work with a Typhoon H.
The CGo3 will never work as well on the Typhoon H as it did on the Q500. You will always have problems with on-screen video artifacts. Recorded video is fine. And the advanced features of the CG03 cannot be accessed on the TH. Some improvement of the on-screen video quality can be obtained by upgrading the camera to the latest CGo3 firmware. You can't upgrade a CGo3 to CG03+ firmware. The upgrade is blocked by the firmware.
The latest CGo3 firmware is here:
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Downloads - Typhoon Q500 4K"