Not sure I am qualified to answer that sort of question... However, I think that something along these lines would be ok in most cases.
This will convert everything except for vectors that use spot colors.
If the file has embedded ICC profiles or Output Intents those will be used. If that's not the case the the defaults above will be used.
It's been years since I last used FFProM so I do not recall what was available there.
As you stated, the Manage Color component has options to convert documents to CMYK. What options are you looking for that are not there?
What are the best settings to convert rgb to cmyk in the Manage Color Preset Node, convert colors area. I want to convert rgb to cmyk but there is no rgb to cmyk conversion in Core like there was in Process Manager. What is the work around.