I bought a Garmin Forerunner 255 watch that I want to use only with Gadgetbridge. There is an old software version on the watch and I want to update it and I don’t want to connect it with Garmin Connect or Garmin Express app?
I have looked for the possibility to do an “offline” update but have not found it. Maybe the community will help?
It does not really answer OP’s question, but there are alternative OSs for consumer devices. I found one: https://wiki.asteroidos.org/index.php/Category:Watches
I know the project and keep my fingers crossed for it, but they currently support very old watch models that are not sporty.