Disseminating math results in an interdisciplinary journal
There are journals in physics that encourage interdisciplinary work and also mathematical physics journals.  I have seen (and written) Physical Review Letters that have many pages of mathematics in a supplement.  See, for example:

Colbrook, Matthew J., Bogdan Roman, and Anders C. Hansen. "How to compute spectra with error control." Physical review letters 122.25 (2019): 250201.

You might do well adding an introduction that addresses the physics and then sending to a mathematical physics journal.

Do not be afraid of covering the same material twice, however.  Perhaps you write the paper as a math paper, and then a separate paper that covers in different words the same material from a physics viewpoint.  You can always call it a review or guide if that makes you feel more honest.  I recently employed that strategy and I hope Referee B feels this is alright.