How can I print varying number of multiple pages per sheet on Mac? (Standard layout / print multiple pages option does not suit)
I wait to be corrected, but I'm not convinced that any 'simple' software exists to do this easily. (I'm wondering whether Safari will split up PDF content for printing in the same way as it does for web pages -- though of course that can cut the page in the middle of a line of text.)
Your best bet is to manually place each PDF page onto the fixed pages of some DTP software, like Affinity Publisher, or Scribus, or InDesign. Unless you've got more than 100 pages, this shouldn't take too long.
Having written a number of python scripts that interact with MacOS's CoreGraphics, it is conceivable that a utility could be written to 'consolidate' the pages onto the pages of a new PDF; though this would require some work.