I shoot a P25+ with the 120mm macro at small aperture regularly (on similar subject matter), and I rarely get dust that causes a lot of work. I try to shoot at f/14 to f/20 if possible, but I'm not afraid of f/22 if it is required. Sure, diffraction creeps in, but there will always be a trade-off with macro. One can focus-stack, if that's acceptable to you, or use T-S tools to take advantage of the scheimpflug principle when diffraction is too great to deal with.
Another alternative, depending on absolute resolution requirements, is to back away a little and shoot at f/14 to f/16 and then crop a little.
In any case, your sensor looks very dirty in your sample. I hope that you can resolve that so that it doesn't dictate your camera settings and composition practices.