Creating graphics in figma for book layout

As per @BobLevine 

InDesign does the same conversion as any other Adobe app, converting images to CMYK in Illustrator you might as well convert them on export to PDF from InDesign.

The only reason to convert to CMYK in Illustrator is if you want full control over the CMYK values, that is that the colours need to be very specific CMYK breakdowns.

If there's no specific CMYK breakdown then leave them as RGB and place in InDesign and then convert on export to PDF.

Where you might hit some issues:
RGB has a wider gamut than CMYK (a bigger colour range) so some colours in RGB cannot be reproduced in CMYK. Resulting in awful looking muddy colours. 

Text that is black at small font sizes, definitely any text that is RGB black will be 4 colour black using all CMYK plates - which for book printing is bad as the text should be just plain Black off the K plate (CMY are separate plates).

If it's digital it probably won't make a difference. 

Let us know if you need anything else.

When you have opened them in Iilusrator and converted them to CMYK, do following:

1. select all artwork. 
   open the Swarch panel
2. in the panel menu create a new color group and take care that global colors area tive
3. Deselect now everything
4.  Click on the first swatch and clean it up and do it with all. Eg after converting a RGB black to a CMYK black yoz end up with 4c black, but it should be k only or color values of.other colors might be odd or to low to be printable. 
   
   

At the end save the file as AI and place it iin INDD. 

The evidence is that most of the "E-Z-2-Use" online graphics platforms produce really lousy export files; they are optimized for use within their own ecosystem, especially for print by the in-system vendor.

The solution is almost always to open and resave them in a genuine Adobe app, be it AI or Acrobat Pro. So your plan is (theoretically) a best option if you prefer using Figma, Canva, etc. to do your initial work.

Furthermorer, the export to PDF for print tends to fix and clean up a lot of problems with graphic elements, unless they are too broken — and the cleanup step should help with that.