Like everyone else has said, I had the same issue with my silicon Apple Watch band when I first got it. It gave my a sweat rash. Eventually my skin got used to it, but I have bought a bunch of other cloth and metal bands to switch out instead. I wear the silicon one when I’m exercising or I need something waterproof, but I mostly wear the others now. I’m not sure how customisable the Garmin is in terms of bands though. There are hundreds of thousands of cheapo bands for the Apple Watch though.

I’m currently wearing my scrunchie band which is great for when I want something more breathable. I usually wear these to bed as well.