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.No boost scroll this time. 
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.
(except for the scroll this time)
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 20 cells
Quest 30 cells
Quest Silver Relics
Quest Gold Relics
Quest LazarBeam's Sword 
If you have any other questions, feel free to reply on this thread.
Anyway, here are the strategies:
Every 8 hours: /pastebin.com/eAwD591z
Every 8 hours, first gems spent after 8 hours: /pastebin.com/qWaUZ5HJ
Every hour: /pastebin.com/2fCVpuuY
Every hour, first gems spent after 4 hours: /pastebin.com/p5cjdBZ7
Every hour, first gems spent after 8 hours: /pastebin.com/Y4YSJ6hG
Also, I put the code i use to create these guides on [Github](github. .com/dorijank. o/ProgressiveIslandsGuider) so anyone can use it as they want.