CMYK vs RGB for IngramSpark and grayscale images

Has anyone here had experience printing a book with grayscale images on Premium paper at IngramSpark? If so, did you get better results following their recommendations to submit CMYK or by submitting RGB images?

I'm producing a book with all grayscale images. On KDP, although KDP says to give them CMYK output, the images have much less of a color cast if I keep them in RGB, and the covers also turn out better with RGB. In both cases, in my multiple test books, PDF/X-1a files yielded inferior results to PDF/X-3 files, presumably because of mismatches between the profile I specified for the CMYK conversion and the one KDP actually uses, which they don't divulge.

IngramSpark warns me that if I use the RGB images for the interior and cover files, their conversion will likely cause a color shift. But my understanding is that IngramSpark and KDP use similar printers and, when KDP's printers are overloaded, they'll have some of the printing done at IngramSpark/LightningSource. And Ingram, like KDP, provides no instructions I could find about what profile to use for CMYK.

My inclination is to go with RGB first, but I'd be grateful for hearing what has worked best for others who have gone down this path.

Thanks -

David