to reply onto the farming case, there are some songs that indeed give a lot of pp with little effort from above average players:
https://osu.ppy.sh/b/161980?m=2 is one case, getting top 200 nets you quite a lot of pp, even when you're at 4.6k pp.
https://osu.ppy.sh/b/55474?m=2 is another example, the intro may be annoying, but its a straight shot to 300 combo, I see why it would give pp because the spacing between the jumps are huge and its pure hypers, but like I said any above average player can easily get accustomed to those jumps and either fc, or even miss once near the end and nets you still some good pp.
https://osu.ppy.sh/b/84485?m=2 the jumps may be annoying but it is definitely doable, only thing I found hard on this is some jump at 369, like I said it gives a lot of pp as well.
although these songs were me experimenting to get under rank 100 with not so much effort, I did in fact move a lot of ranks so far from a few maps. And I'm considering these songs farmable since like I said above average players can do, or fc the songs that I listed with few retries
it does seem massively spaced maps gives a lot of pp too which may be concerning, since some of the maps have a few spots that are considered hard to pass, but require a few attempts to actually get used to it and pass massively large spaced notes.