Bought the Karoo2 when it was super discounted ($250 iirc) because the battery on my 520 was getting pretty dicey. Otherwise, still in the Garmin ecosystem for watches. It's generally been pretty good, but the requirement for internet connectivity for search with the navigation (and no easy way to piggyback on your phone connection) definitely feels like a thing they need to fix on their next version (iirc the 840 can do totally offline address, but not POI, search). The battery life has been fine for me. Otherwise, it's been generally decent so it'd be hard to see what would be a compelling hardware upgrade (unless, to your point, they move to a newer version of Android that only works with newer hardware).

When it is time to retire the K2 I'm looking to return to Garmin. I loved it when I could side loads the apps by running the android OS that's not supported anymore put a damper on things. Also little things like all power data fields read stop reporting data when at a coffee stop. It's like they've never had a coffee shop and wanted to quickly look through and check in on your TSS, KJ, etc. not a total loss but still not a choice I agree with.