steam-rom-manager VS HeroicBashLauncher

Try using an older version os steam rom manager from here. Try 2.4.11. I had a problem where steam rom manager wasn't parsing images and shifting to an older version helped.

That's correct. Thats mainly because steam rom manager exists, but if there's interest I'm happy to look into including non steam games in the "Games" section

It never worked so greatly to begin with. So try https://github.com/SteamGridDB/steam-rom-manager instead that happens to be actively developed and updated just today.

Can you utilize addtosteam.sh with this similar to how heroic did?

I've previously used Heroic Bash Launcher for that. It generates a script that opens the game in exactly the same way Heroic does, and you can then add the script as a non steam game.

The EUREKA moment came from a thread for CIV IV on a random Heroic Launcher Github thread. https://github.com/redromnon/HeroicBashLauncher/issues/38

Works for me with Lutris shortcuts and some native Linux games I have installed. Herioc shortcuts needs an extra step using Heroic Bash Launcher

You're probably better launching through heroic and not directly using proton. This project looks like it might add all your heroic games to steam automatically: https://github.com/redromnon/HeroicBashLauncher

Heroic Bash Launcher

Use Heroic Launcher (find in desktop mode via discover (flatpack)). Install Windows versions to take advantage of Steam/Proton. Sync saves using Heroic Launcher too (be careful the feature is currently in Beta, but working fine for me in D:OS). Install the shortcut in your steam library using Heroic Bash Launcher, job done.

For heroic installed games, i used https://github.com/redromnon/HeroicBashLauncher to add steam shortcuts for individual games.

But for Heroic, afaik you need this, though I am still working on the guide and can't 100% confirm.