Installed TFix 1.27 (https://www.ttlg.com/forums/showthread.php?t=134733) with the following script:

#!/bin/bash

set -euo pipefail

export HERE=$PWD

export STEAMPATH=~/.steam # You will probably need something else
export WINEPREFIX=$STEAMPATH/steam/steamapps/compatdata/211600/pfx
export WINEPATH="$STEAMPATH/steam/steamapps/common/Proton 5.0/dist/bin/wine64"
cd "$WINEPREFIX/drive_c/Program Files (x86)"
ln -s $STEAMPATH/steam/steamapps/common/thief_gold/ .
"$WINEPATH" "$HERE/TFix_1.27.exe" # to apply TFIX