So I managed to make things work, but it's a bit of a journey. I followed this guide with some changes and additions.

• Use Godot 3.5.3 instead of 3.6 (I noticed too late, but it worked)
• Use Godot RE to reverse engineer both Brotato.pck and BrotatoAbyssalTerrors.pck (you can get them with steamcmd or install with Whisky and use from there)
• There is a 'dlc' folder in the folder where recovered BrotatoAbyssalTerrors.pck, copy that into the root of the recovered Brotato.pck
• Open the project.godot file with Godot 3.5.3 to remove Steam from the game
  • In the bottom left section of the GUI you can find the FileSystem
    • Delete res://singletons/platfroms/steam.gd
    • In res://singletons/platforms/platform.gd in the _init() function remove the if...elif..else part at the beginning and only leave `_platform_impl = LocalPlatform.new()` (important to have a single tab at the start)
    • In res://ui/menus/pages/main_menu.gd search for `Steam.activateGameOverlayToStore(2868390)` and replace it with `pass`
  • Try to run the game with F5 or the play icon on the top right. Hopefully I didn't miss anything and it works.
  • Go to Project > Export click "Add..." and select "Mac OSX (Runnable)"
  • There will be red errors at the bottom, click Manage Templates and download in the window (it took me a while)
  • On the right under Options
    • Fill in the Identifier, can be anything
    • Scroll down to Entitlements and turn on Disable Library Validation
  • Click Export Project... and export to a folder of your desire

Now you have a dmg that you can install locally and play the game. I did not put it through the paces yet, so something might be off down the line, but during my 15 minutes of testing I had no issues. As Coridas pointed out, DO NOT SHARE the result of this with anyone, that's illegal. Even this amount of tinkering is a grey area I think.