I think the only sports watch company that has an app that is worse than Garmin's is Polar. I have used Garmin devices since 2010, but their UX is (as you say) pretty quirky. They changed the UI/UX of the gpsmap H1 to look more like a smartphone, but it is still weird. Another issue is that their software has been very buggy the last few years, with software only stabilizing 1-2 years after the release. One the largest external source for Garmin information (gpsrchive) has actively recommended against purchasing the H1 because it has been so buggy. Similarly, earlier firmware releases of the Fenix 8 had a lot of issues. Also a lot of functionality of older units hasn't been implemented yet. These are often not small bugs, but of the type, "oopsie, your device froze or rebooted while you were navigating". They basically released an alpha version as a final product.

I don't know about UX, but I've had my Coros watch for a few weeks now and I didn't find it hard to understand. I think it's much easier than when I first learned to use a Fenix watch. It misses some Garmin features though that I'd really like to see like off-course rerouting. But like I said above, they have been adding features at a good pace and a drastically undercutting Garmin on price (most watches less than half the price of the closest Garmin watch).