Use a simple script to run the installers in the same container as the base game:

#!/bin/bash

export STEAM_COMPAT_CLIENT_INSTALL_PATH="$HOME/.local/share/Steam"
export STEAM_COMPAT_DATA_PATH="/$HOME/.local/share/Steam/steamapps/compatdata/*prefix number*/"

"$HOME/.local/share/Steam/steamapps/common/Proton 10.0/proton" run "$HOME/Downloads/StellarisExp1.exe"

Just change the relevant parts to run the other installers, e.g. the installer name and location and the prefix number to match what Steam has generated