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You can request re-indexing of a single page via Google Search Console.  It would seem to me you could use this feature to experiment with on-page changes to see the rank change to determine which changes have the most effect.  For the sake of this thread, lets temporarily forget that the relative importance on various on-page factors has already been reverse engineered to a degree so we already have a general idea to som extent. It would seem to me if I were Google, I would introduce either a random delay period, or, temper rank change after reindexing. What I mean by that latter point is say a reindex takes a page from position 20 to 10.  If it is 'tempered' so to speak on Day 2 after reindexing it might be at 18, day 5 it's at 16, day 7 it's at 16 until it reaches the actual "real" rank.  Both the delay and or the tempering of rank change would make it difficult more difficult to reverse engineer relative importance of on-page factors. OR, does Google realize there are large SEO firms doing SEO over several years for many sites that can examine aggregate data to determine these factors so Google doesn't delay (aka sandbox) or temper rank changes due to manual re-indexing?