Hello, I also had trouble with the measurement. It almost drove me to white heat, including the battery changes you described. However, at some point I realised that it was the belt that was causing the problem and not the sensor.
I have now been travelling with the new strap and the old sensor for about a year without any problems.
Incidentally, I also had a Suunto 9 and now a Vertical.
Unfortunately, I don't know of an alternative to the Suunto sensor, but try a new strap and then wash it carefully by hand after training.

I was just surprised to see that the Polar sensor is also supposed to be compatible.
Polar H10 size M-XXL

"The Polar H10 receives software updates via the Polar Beat app; it transmits the heart rate to sports watches (e.g. Polar, Garmin, Apple, Suunto) and fitness apps, e.g. Polar Beat, Strava, Nike"