ANT+ is a broadcast protocol. The HRM - whether using your forerunner or a chest strap - broadcasts and the Edge listens for the broadcast data from the sensors it's been told to listen to (i.e. paired). So either your Forerunner is somehow stopping its broadcast or the Edge is somehow stopping listening. Kinda tough to discern which is the case. Either way this is a bug. I use my old fr235 with my edge 830 for HR and it remains rock solid even on 40mi rides with stops.
If you've not yet made sure everything's up to date on software and if you've not unpaired / re-paired the forerunner & edge, I'd do so first.
As for a different/new HRM - speaking from experience of many years running with chest straps... they can be a pain in the azz. especially in cooler months. You might wish to consider replacing the forerunner with a dedicated optical HRM armband. See
https://www.dcrainmaker.com/2019/04/...tickr-fit.html for some ideas.
Edit - as for power meters. Consider the Stages or 4iii crankarm mounted options. I chose the 4iii Precision 3 and have been happy with it. Choosing one vs other largely came down to which was available preinstalled on the crank arm at the point I was ready to buy.