So, I made the best strategies for spending gems if you want to reach a certain node. First, I'll explain how this works:

For each quest, there are several possible strategies with various amounts of time necessary.

For the strategies which collect every 8 hours, I'm not considering the boost scroll you get for free(because to take advantage of it you need to collect after 4 hours, which you obviously can't if you collect every 8 hours). For the strategies which don't collect every 8 hours, I'm assuming that you do perfect collections and take advantage of the boost scroll.

I'm considering the amount of time required to get items once they are in the pool as 0. Also, this time some nodes require PvP; I'm assuming that you start every node with 3 PvP attacks. (this means that if you really want to get the monster for the minimum number of gems you might need to save up attacks)

How the strategy works: For each quest, the strategy tells you the number of gems to spend on that quest. Note that quests which only require 1 gold item can be completed almost instantly so they aren't included.
It also says after how much time is required to get you to this gem cost with perfect collections, or, if the time isn't affected by the island boost and it is divisible by 8 hours, the number of collections. 
To save time, I put all the strategies of the same type in one place; you can search(ctrl+f on pc) one of these things:
Quest 15 cells
Quest 20 cells
Quest 65 cells(r0)
Quest 60 cells
Quest 60 cells(r1)
Quest 75 cells
Quest 75 cells(r2)
Quest 130 cells
Quest 160 cells(r3)
Quest 180 cells
Quest 190 cells(r4)
Quest 320 cells
Quest 420 cells(r5) 
If you have any other questions, feel free to reply on this thread.
Anyway, here are the strategies:
Every 8 hours: /pastebin.com/CAgsjRsm
Every 8 hours, first gems spent after 8 hours: /pastebin.com/naCCkC3H
Every 4 hours: /pastebin.com/dFYbevaL
Every 4 hours, first gems spent after 4 hours: /pastebin.com/ydqA257V
Every 4 hours, first gems spent after 8 hours: /pastebin.com/XzSrfBpD
Every hour: /pastebin.com/mnCXFhsL
Every hour, first gems spent after 4 hours: /pastebin.com/ZkFBLC75
Every hour, first gems spent after 8 hours: /pastebin.com/pV8JmewU
Also, I put the code i use to create these guides on Github so anyone can use it as they want.
Also, this is the first time the boost scroll is a reward so it's possible that some of the numbers are wrong; if you notice that, please inform me on this thread.