If you have a functioning bike computer, is there a reason that you do not want to use a free standing HRM in addition to it?

I am quite happy with a Sigma HRM, wrist watch style. I use it with an handlebar mount from a previous HRM that I had, but you could certainly wear it like a wrist watch. I bike tour and leave it on my bike out in the weather, have not had any problem with it getting wet. All it gives me is HRM (plus stopwatch, time of day, stuff I do not use), no calories or averages, or anything like that, just gives me the data that I want which is current rate in big numbers that are easy to read.
https://www.sigmasport.com/en/produk...d-series/id-go

Amazon has it
https://www.amazon.com/Sigma-Sport/dp/B07M8VTFD2

Uses the older style chest strap that you do not have to pair after a battery change. I had several older chest straps from other HRMs which you might not care about but I saw as a plus.