Gadgetbridge is an Android (5.0+) application which will allow you to use your Pebble, Mi Band, Amazfit Bip and HPlus device (and more) without the vendor's closed source application and without the need to create an account and transmit any of your data to the vendor's servers.

Website: https://gadgetbridge.org
Codeberg: https://codeberg.org/Freeyourgadget/Gadgetbridge
F-Droid: https://f-droid.org/packages/nodomain.freeyourgadget.gadgetbridge

EDIT: I just realized, that Samsung wearable devices are using WearOS, which is unsupported by Gadgetbridge. So your best options are either to stick with the proprietary apps or to check out devices, that have good support in Gadgetbridge. For example bangle.js, Garmin, etc.

it doesn't support any Galaxy Watches at the time I'm writing this, but if you end up picking another device, look into Gadgetbridge and which devices it supports: https://gadgetbridge.org/

As far as I know it can't be done.

Look at the watches on here: https://gadgetbridge.org/gadgets/

I have Amazfit Balance and it works very well, all the data is locally synced to my phone, none of it goes to Google or China.