The 945 HR is pretty good. But, it depends a lot on the individual. Some skin just doesn't work well with some wrist HRM. I would give it a try and just observe how it tracks for you relative to your old chest strap. And, look at your charts after the run to look for anomalies. If you don't love it, you can always buy a new external HRM.

Edit... Thought more about your "broadcast HR" comment. If you want to use the 945 HRM for cycling, that might suck. All of the wrist HRMs I have tried are crap on a bike. I suspect it is because you have your wrist muscles tight while holding the handlebars. I would still try it, but you might not like the performance if on a bike.

Wrist-based HR is highly variable on the person. Wrist-based HR works really well for me for running and generally day-to-day HR tracking. For others though it's a complete crapshoot. From what I can tell though is it's generally considered not so great for cycling.

Personally I have always hated chest straps and have been in the category of arm straps since they first became available many years ago. I'm currently using Wahoo's version of the arm strap and love it. Works great for me.

Fenix owner here.
Wrist HR is perfect until I start doing sports. Than it shows phantasy values.
I use the chest-strap then.

Additional advantage of a chest-strap: sometimes (in the winter) you want to wear your watch over your shirt sleeve.

Yeah, I've noticed the wrist based HR getting weird while riding. I always figured it was from me choking the crap out of my bars while riding in city traffic. It's always been fine for running and swimming if its tight enough.

I do hate the chest strap, it constantly slides down due to my lats. But if its that much more accurate, I'll deal and buy another one.

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Strava

I made the jump from the chest strap to the arm-based optical HRM several years ago. I absolutely love it; I hated my chest straps. I started with the Rhythm+, then the Rhythm24, and now the TICKR FIT.

945 built in optical HRM is terrible for bike-run and swim.

Dedicated optical HRM like Rhythm+, TICKR work extremely well and have far superior longevity compared to chest-based nonoptical straps. My Scochse Rhythm+ is 4+ years old now and the optical sensor works as well as it does on day one. I've gone through 4 strap changes, one charger change (each costs $15) but once you find the spot where it works well on you, it's all go.

Stopped using Garmin straps after they kept dying in the 6-12 month range. Literally killed 6 of them, of all types, soft and hard, despite washing them, changing out the puck, etc.