Hm we had a similiar issue a year ago, without a solution
https://dietpi.com/forum/t/waveshare-7-9inch-dsi-lcd-touch-screen-cannot-display-landscape-please-help/18282/7
Also in their docs they say, they only support Pi OS / Ubuntu / Kali and Retropie, but no plain Debian. Maybe you can ask Waveshare support for help.