When I read the TMC2660 datasheet, it sounded like the RNDTF bit doesn't affect stealthChop mode, because it's only discussed in the section about constant off-time mode. So if you want to try setting it in stealthChop mode, you will have to calculate the value of the whole chopper control register that you want, and set it using the C parameter of M569.
The TMC2660 doesn't support chopSync or stealthChop. It does support coolStep, which reduces motor noise at low and medium speeds. It's disabled by default, because (like stealthChop) it increases the likelihood of skipped steps if the nozzle hits a blob or overhang.