Another update: tried a hacky workaround and edited the Mac config file to allow Xcode 16. It did get work to an extent, I was able to run./GenerateProjectFiles.command without error, and all the folders appeared which was huge!. But Product->build failed.
Then I found in Reddit:
Unreal Engine 5.3 doesn’t support Xcode 16.2 or macOS 15 (both of which I have)
I can delete Xcode 16 and try brew install Xcode 15, but on the App Store that is not obvious how to install a previous version, I cannot change my OS backwards.
If there are any UE staff reading this, is there any way to build UE5.3 (or 5.4 or 5.5) build-from-source on Sequoia15 and Xcode 16?