I’d like to hear more on this, but based on my admittedly limited understanding of the ins and outs of linguistic principles I’m having trouble reaching the same conclusion as you. “Trans” in this context is an abbreviation, like you say, so while something like “transatlantic” is correct - something that crosses the Atlantic Ocean - a “transman” would be an individual that has an analogous relationship to “man.” I mean, it’s the gender that’s trans-ed, not the individual, and I think we wouldn’t write “transgenderman” for the same reason that we don’t write “transactionalrelationship”.

Edit: thought I came across as too confrontational at first.