I finally found out how to get my camera into 120fps HD mode. It is only accessible, once I have gone into the menu, and switched the System to a specific HD 30p codec. But I'd love to be able to switch back and forth quickly between 4K 60p and HD 120p, and that menu method is clunky and slow. The camera allows me to assign a shortcut button to the super slow mode function, but it only works if I am in that codec.

Does anyone know a quicker way to switch between my favorite frame rates? Can I use scene files or something similar to preset?

Hi Matt,
The only way I can think of achieving this is using Setup files. Save the 4k/60p setup. Then make all the adjustments for HD 120fps and when happy, save this setup also. Then you can Load the appropriate Setup file when required.
Best Regards
Graham
ps. The X1500 is a sibling of the Panasonic CX350, with a similar operating system. Barry Green's excellent (and free) PDF book is a must-read if you haven't already come across it. https://na.panasonic.com/us/CX350book