After trying to play the game again nearly 15 years later I've ran into a few different problems and it took me quite a bit of research, frustration and testing to get it all to work as intended, so I figured I'll put all those fixes in one place as I haven't seen something that includes all the current issues and solutions.

Fixes 2 and 3 are taken from their respective creates, they deserve all the credit for them, if this helps you, please go and give a thumbs up to their guides.

This problem as far as I understand relates to the game being unable to use multiple core CPUs, the fix :

• Download the Windows ADK for your OS - you can find it on official Microsoft page here https://learn.microsoft.com/en-us/windows-hardware/get-started/adk-install
  
• During installation, it gives the option to select multiple tools; the only one needed is Compatibility Administrator included in Application Compatibility Tools feature.
  
• Once installed, open the Start Menu folder named Windows Kits, and launch Compatibility Administrator (32-bit).
  
• Select the new database under custom databases.
  
• Click the database menu > Create New > Application Fix.
  
• Provide the full path of DAOriginsConfig.exe and give it a name of choosing.
  
• Skip past the compatibility mode options.
  
• In compatibility fixes select ProcessorCountLieForHybridCPU, and also potentially ProcessorCountLie.
  
• Save the database from the Database menu and give it a name when prompted.
  
• Select File > Install.

This let me open the Configuration tool from the launcher, save the settings and actually start the game.

After the decommissioning of the Bioware portal it's impossible to download the DLCs within the game any more.

Thanks to Dart of Desparity's comment - the DLCs can be downloaded from https://dragonage.fandom.com/wiki/Downloadable_content_(Origins)

Solution (piggybacked from SoulsBorne's guide :

What you need to do:

• Go to your Dragon Age Origins install folder - you can find it by right clicking the game in your library and going to Manage -> Browse local files
  
• Open "redist" folder
  
• Run DAUServiceSetup_Steam.exe file
  
• Run DAOU_UpdateAddinsXML_Steam.exe file

After you run your game again it will have all thje DLCs - missing some premium DLCs, I'm not sure if there's a way to get those, but let me know and I'll add a how-to it here

This last part is perhaps the most annoying one after you've done already everything else, but this too was already solved in 3xALucky's guide here

just in case something happens with the above, I'm also hosting the fixed exe here
https://drive.google.com/file/d/19-L899eUVfnN7zR-gzh3GBZoQAhmrGQG/view?usp=sharing