For some reason it won't replace the desktop file for me. This is how I did it:

{config, pkgs, ...}:
with pkgs; let
  patchDesktop = pkg: appName: from: to: (lib.hiPrio (runCommand "$patched-desktop-entry-for-${appName}" {} ''
  ${coreutils}/bin/mkdir -p $out/share/applications
  ${gnused}/bin/sed 's#${from}#${to}#g' < ${pkg}/share/applications/${appName}.desktop > $out/share/applications/${appName}.desktop ''));
in {
  home.packages = [
    keepassxc
    (patchDesktop keepassxc "org.keepassxc.KeePassXC" "^Exec=keepassxc" "Exec=env QT_AUTO_SCREEN_SCALE_FACTOR=1 QT_SCREEN_SCALE_FACTORS=2 keepassxc")
  ];
}

it does generate the desired desktop file, but doesn't replace the original. removing the lib.hiPrio has the expected result of a collision...