The umbrella header contains both import and export features so takes some time to compile. A prebuilt binary xcframework (for iOS & Catalyst only) is wrapped as swift package, and works in XCode 12 Beta only. This approach is experimental and has some issues with code signing.
https://github.com/assimp/assimp
pod 'Assimp', :git => 'https://github.com/ndreca/Assimp.git'
.package(name: "Assimp", url: "https://github.com/ndreca/Assimp.git", from: "5.0.2"),