Paul, no real help here. Good luck in your quest.
The only set of mediums (54-12) that I have ever used were a medium gauge twelve string set that came on an Emerald carbon fiber twelve string. Tuned to standard pitch, the tension was a whopping 335 pounds (versus 250 pounds for a typical 47-10 light gauge set). Boy was that a handful to play, even though that guitar could actually handle the tension. Those were an uncoated set of D'Addario EJ-37 and were removed after a day. (Odd factoid: if you removed the octave strings, you basically end up with a typical light gauge 54-12 set). You are tuning down to about the same overall tension as a light gauge set, and the mediums you seek would be correct. But there was apparently not enough demand for Elixir to keep that set in production.
All I can suggest is
Just Strings or
Strings by Mail and see if you can build your own sets using bulk strings. On my twelve string, I use GHS
White Bronze and that nickle set lasts almost as long as coated strings for me. But those are not available in the heavier gauge either. They don't work well tuned down below D-d.
https://www.juststrings.com/ghstwelvestringguitarwhite.html
