I would say a medium or hard brush would be the best option for cleaning your teeth.
Soft bristles would bend on contact with your teeth so you’d only be brushing your teeth with the sides of the bristles which would be pointless.
I don’t think the firmness of bristles is what damages your gums or teeth so much as the pressure applied when brushing.
Most people use too much pressure which is abrasive to gums and teeth. I guess the firmer the bristles the less pressure you need to use.
Best option is to use an electric toothbrush and to floss. I changed to electric 6 months ago and the difference is amazing and instant.
Failing that, to use less pressure, hold your brush closer to the end (furthest from the bristles).
Well that’s it for my first ever post on a message board!!