I recently acquired a used FL-600R flash and so far am very pleased with using it on my M5iii. Giving it a quick test with some out of window shots - outside was very bright, inside was dark. I was favorably impressed with the results - both bright and dark areas were (mostly) correctly exposed. More “normal” shots were correctly exposed.
I still haven’t figured it all out yet - don’t have a user manual. But one feature was totally unexpected - with the flash mounted on my M5iii, when I turned on the camera, the flash was also turned on and vice-versa! Never had that happen with Pentax or Canon cameras and flashes.
I’ll be keeping the flash!