My Forerunner 235 died a few days ago, shame as its been great for 4+ years but lucky that its black Friday!
I want to stay Garmin and have narrowed it down to a budget including the following 4 but they’re all so close I can’t work out which one is the one I should get. #firstworldproblems
I should add I am primarily a mountain biker, that’s my focus but I do like metrics, health metrics, gym, running etc etc.
Forerunner 245
Vivoactive 4
Instinct Solar
Forerunner 645
So which is it?! 
I’m happy with my 245, and use it on the bike mainly off road where I don’t have a power meter. I don’t think it supports power meter recording.
I only occasional run, but I do use the Heart Rate Tracking (resting heart rate) and Sleep Tracking.
Optical Heart Rate works ok for me on the bike, except when I’m on the rivet on the trainer. Commuting (in the days before remote) I just use optical, but on weekend gravel I normally put on a strap. It does broadcast HR too, but again, on weekend spins I normally put on a strap.
I can only comment on one, well its predecessor the Vivioactive 3. If you are not too bothered about training (power etc) it looks good IMO and was the right price for me. I use my Garmin head unit for training so it ticks the boxes for me.
