I just use Garmin excellent fenix 6 that provides all the data. I do have polar HR chest strap but I hate wearing it. You learn to know where your heart rate is based on how hard you are working. I like the watch a lot as it records everything and then auto uploads to Strava. It’s very easy to glance at you HR on the display
I bought this one in Amazon:
It is pretty inexpensive and has dual connection to my Fenix 5x using Ant+ and also to the bike using bluetooth. You won't lose the exercise recording in the watch while still seeing your HR in the flyon display.