I've been using Gadgetbridge, installed via F-Droid, to interface with my Xiaomi Mi Band 4 for years now and can only recommend it. For my device, it's 100% feature-complete, it's VERY polished design- and UX-wise, and it comes without any ties to any vendor's data-siphoning cloud services. You can even set it up so that it works in parallel with your vendor-provided app, by not deleting any data from the device it fetches, if you're yet uncertain you actually want to use it as a replacement for your current app.
VERY polished? I think not. It suffers from lack of decent UI because it's built by hackers (in the most positive way) who wanted a focus on privacy.
Here's one example: track a run/ride and take a look at the session details. You'll see your average speed graphed out in bar chart form... There will be hundreds of bars stuffed into a two inch wide graph. Not the most user friendly.
As always, the answer with Open source is do it yourself, or sponsor someone else who can :)
Besides, for the privacy benefits it provides, its definitely good enough. You can also export data and have fancy graphs be shown with minimal effort.
Free software at its very best, really!