Hey Jimmy,
If you like the tapes on your mustang and want to change things up, i would recommend asking Jake to put on either the DR Hi-Beams, or the Sunbeams, which are mellower than the Hi's, but not as mellow as flats or tapes.
I had a Serek Midwestern built in 2020 and actually just sold it a couple months back, because i wasn't playing out enough to warrant having such a sweet and costly bass, but i would regularly switch out between Hi-Beams, LaBella Flats and the Sunbeams, depending on the project and song (if it was a recording session). Mine had a B90 in the middle position and a Curtis Novak ES-BSx in the neck position, which was super mellow.
Trying to find short scale strings to fit the Serek will be easier than finding a medium scale string that you like for the Mustang.
That being said, i recently purchased, and have yet to try them out, a couple sets of LaBella's Mustang strings for my Vintera II. I bought their Pure Nickel Rounds ((760PN-MUS)and the Super Polished (760SP-MUS) sets, straight from LaBella in NY. These are all made specifically for the string through body Mustangs. Once this set of new Hipshot tuners arrive, i will switch out the flats for one of these to test out and can report back, as to which i prefer, or at least, what i like and dislike about each of them.