I looked into this matter again. So ColorSync remains a good choice for post-editing profile embedding. Preview can be used for soft-proofing post editing. GIMP appears to offer soft-proofing while editing. So A possibility is to do most editing in RAW Power, export as 16 bit TIFF ProPhoto RGB or AdobeRGB and then check the expected print output including out of gamut colors in GIMP. I will try that when I will have my next larger format print made.