Problem solved - Looks like it didn’t like me setting the Type to External in .Build.cs
If anyone else stumbles by this, here’s my assimp module as a reference
Build scripts/module: https://github.com/UnrealMKB/UnrealMKB/tree/model-loading/UnrealMKB/Source/ThirdParty/libassimp
Git submodule for assimp is located in /ThirdParty/assimp of the repo
(If I forgot to update the link and that link leads to a 404, try this: https://github.com/UnrealMKB/UnrealMKB/tree/master/UnrealMKB/Source/ThirdParty/libassimp)