If you are afraid to open the card and add a pad you can lower the ram settings. plus hit the backplate with a fan
In my opinion, 91ºC is good enough to RTX 3090 without any changes on thermal pad
Many people struggle to keep under 100ºC
I like and always recommend to put a fan blowing directly to backplate, at least 2ºC you can reduce and something like 89ºC is safe enough to run for the long term.
It's cheap and don't void the warranty, specially of a 3090
The idea of changing the thermal pad is excelent and will reduce temperature, but many brands will refuse warranty if you open the card