Hi, sam028,
I'd like to share some photos of one of my ZMR250 builds, they may be helpful to you...
All GPS / INS13 functionality works fine, no toilet bowls, no oscillations.
I use this frame for short to mid-range fpv and sometimes have to rely on RTB if signal is bad/lost. PositionHold, AutoLand are ok.
Completely solved the problems with INS13 / AuxMag on my ZMR250 frame by carefully twisting ALL high current wires. TheOtherCliff and f5soh already pointed that.
Furthermore, I've mounted the battery plug, primary power wires to the PDB, current sensor, BECs, Filters, LED driver, etc. between the bottom two carbon-fibre plates of the frame. Just used additional 5mm & 8mm spacers (this build has 3mm motor arms) to reach 8mm separation between the bottom plates.
AuxMag is integrated with GPS module (Beitian BN-880) that is placed farthest possible from motors and their wirings (on the top of the HD camera heatsink / mounting plate). I found that using GPS mast is not necessary for this particular build. Just had to calibrate the AuxMag for each type/size of battery I use due to its proximity to the AuxMag sensor. No such a problem to upload specific AuxMag calibrations when changing battery type, as the PID settings and FlightBattery settings should also be changed in this case.
Completely reworking already built frame is painful process so you may just try to twist the motor-to-ESC wires and the ESC-to-PDB wires, not touching the frame structure. If you put the AuxMag sensor on a mast, it should be sufficient.
You may submit some photos of your build here, this may help someone to give you more precise guidelines.