I was going to recommend the Gadgetbridge app, but it seems to have little or no support for Fitbit. I does support hundreds of devices, though. I used it extensively with a Mi Band 3, but have yet to try it with my Garmin.
I know Gadgetbridge as it's the official way to use my open-source BangleJS smartwatch. I would have got the rest of the family BangleJS watches too, but they don't like the style of them.
rjmunro|3 days ago