Honestly, the HARDEST part by far is going to be treasure. Because Pathfinder gives out gold like it's going out of style.
And at level 20, four CR 5 monsters should be an alright fight.
One trick on encounter building I picked up from Mike "Sly Floursh" Shea is that a monsters of a CR equal to 1/3rd to the level of a PC is a good one-on-one challenge.
So a party of 4 PCs can have a decent, challenging fight with four CR 1/3 monsters. And at level 3, four CR 1 monsters will be a good fight. And at level 12, four CR 3 monsters will be okay. And at level 20, four CR 5 monsters should be an alright fight.
Give or take. Depends on how badass the party is.
Thinking back to Rise of the Runelords, that starts with a few fights with goblins. 4 goblins is a good, easy battle. Simple to make one a "bard" by giving it a bardic inspiration trait and maybe the vicious mockery cantrip. And liven it up with torches and vials of oil.
Then have a "boss fight" with a goblin boss on a worg, maybe with a goblin or two as support.
Simple.
It's as a baseline and the xp gained works out to being close to the encounter math in the book.I think you are smoking the wacky backy there my friend.
We used to take on multiple deadly X5 fights in a day (using the xp multiplier) or CR +6 or +7 above party level and I think our record is 11 encounters. 6 fights of deadlyX2 also works depending on the composition of the party.
Its roughly CR +1/-1 level for mano a mano challenges (deadly), a level 3 or 4 fighter can maybe take a veteran or a cleric can use a hold person spell and win that way depend on things like optimisation levels, rolled stats, 3pp stuff etc. Vampire RAW are a deadly fight for a level 7 party and they are CR 13 (and not that hard either). I had a level 5 halfling dual wiled basically solo some CR 5 stuff today via an action surge and 3 attacks every round.
Kinda...A previous comment about Pathfinder handing out money I think its similar to 5E. HoTQ queen has a big dragon horde at level 7 and a few of the other adventures also hand out a bit as well. Might not be quite as much as PF but its not far off and money mostly doesn't mater in 5E anyway you could give them 500k gp by level 10.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.