Fact of the matter is that you shouldn't have to buy a new strap when the factory one is causing physical damage to you. I plan on contacting my carrier today to see if they'll send me one on behalf of Samsung.
Its not the strap that's the issue. Its caused from the sensor. I tried wearing my watch a little looser and it helps a bit!! My rash is the exact same as yours!! It drove me mad... i hope u get it sorted 😁