I’ve been using a Polar H9 on my fruity phone for over a year now and never had any major issues other than it not auto connecting anymore if I didn’t use it for like 5 days or more.
Never really liked watches so never considered it, I get all my stuff fine on the phone.
Yeah HRM training is very different, if it gets too high, slowing down or even walking might be necessary.
I also echo the distrust in wrist based HRM, you really want a proper HRM strap and they are pretty affordable compared to watches.
A Polar H9 for example is very high quality and easily less than US$80.