What a pleasure to read something simple as well as lucid like this:
http://www.wbabin.net/physics/nahhas18.pdf
Congratulations! I guess the author is aware of possible construction of almost infinite number of kinematic quasi-TRs which all would be signal-velocity dependent. And all of them would be falling apart on its exact v-limit. Take for example such an hypothetical sound-TR...
Of course, next challenging step even for this author would be to reconsider light motion as general idea. Also, it is about time for "academia" to rewrite old textbooks almost entirely.
Blessings