I use the Fossil Collider HR with Gadgetbridge and it’s pretty great. Three are some downsides though:
The set-up is a little bit of a hassle. You need to get the Collider’s secret key, and that requires installing a jailbroken version of the Fossil app
Any changes you want to make to the watch display have to be made in the Fossil app, which means you have to install that app and do a bit of a runaround whenever you want to change something
But aside from that, it’s been a good year with my Collider HR and Gadgetbridge