I never tried Scalos or proDAD in the past but like said before, Scalos is presented as a Workbench replacement (like that last versions of DOpus) and not as a real OS, that would boot the Amiga, managing every hardware component and supplying its own software layer without any calls to AmigaOS, exposing an API. In fact, a true AmigaOS replacement.
As for the linux, well it's a port of an existent OS to the Amiga, not anything new in concept.
Something like building an OS from scratch would for sure be difficult to implement as a hobby - take AROS for an example - but I wouldn't be surprised if an OS developer enthusiast did some attempts. It seems if there were such cases nobody published or announced their attempts.