I kind of had narrowed it down to the Bryton 310. There is a $125 combo of the 310, cadence sensor and chest HRM.

But now I'm getting second thoughts on if I want a chest HRM or should look for a watch HRM. i realize the chest HRM is a bit more accurate. But a watch I may wear all the time and also for other activities can just see what the HRM is unlike a chest strap i need the phone to "see" HRM. I'm also not sure if i will like the "strapped" feeling on my chest.
I'm also exploring to use the phone, just don't have a neat and safe mounting solution. My S9+ is huge in its case. i have one of the cheap handlebar devices i used on a trainer bike to watch videos while training, but no way i trust that thing to hold my phone riding.