FYI, I started working on a SPDX->DEP5 and DEP5->SPDX converter tool, the code (or rather a basic concept) is here [1].
My goal is to produce an internal representation that collects copyright information on a per-file basis, and convert between SPDX/DEP5 and this format. Regards, Stephan [1] https://github.com/stephanlachnit/dep5convert