The .ZIP files from samfrew must not be installed via sideload or via TWRP 'Install'.
Samfrew’s G960FXX......-202101.........zip must first be unzipped. You’ll then receive a maximum of five files (AP, BL; CP, CSC, HOME_CSC) with the file extension .tar.md5.
The four files (AP, BL; CP, CSC) .tar.md5 must then be installed via Odin3 or Heimdall.
Since you are using a European Galaxy S9 ‘starlte’, I recommend using a ROM for the BTU (United Kingdom) or NEE (Nordic Europe) region. This will also work in the USA.