Firstly, if you don't know how progressive islands work, refer to this post.

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 monster, there are several possible strategies with various amounts of time necessary. I'm also putting some strategies that take longer to complete than the island lasts because, if an island boost happens, that effectively increases the event time by 2 days(if you're active and don't sleep. If not, it will be a bit less of an increase.)

-To find the strategy that suits you, think about how many times will you collect an entire pool of items in the 7 days of the island, and multiply that number with 8 hours to get the amount of total time you want on your strategy.

-I'm considering the amount of time required to get items once they are in the pool as 0.

-How the strategy works: for each quest that can't be completed instantly, it tells you when to spend gems. I'd recommend you base yourself off the gem cost, but you can also divide the time by 8 hours to see how many times you need to collect the complete pool. If there is some remainder, that means that the strategy recommends you to collect an incomplete pool. The most common example is when the remainder is 4; at that time you have regenerated exactly half of the pool without any waste.

-If you have any other questions, feel free to reply to this post.

Anyway, here are the strategies:
-Predagelum(rank 2): https://pastebin.com/KC5z7ATU
-Predagelum(rank 1): https://pastebin.com/TjAd60HE
-Predagelum(rank 0): https://pastebin.com/T42kVjkF
-Amphidator: https://pastebin.com/7ee7sGcU
-Alpha Cliviast: https://pastebin.com/ZueYJhtR
-Hirondeleor(the majority of players will most likely get him without spending gems, but i'm still gonna put up the strategy for those who aren't able to be active enough): https://pastebin.com/UjpmE9hE

EDIT:
I also made a version which only counts log ins every 8 hours; if you don't want to bother with partial pools, this is for you.
-Predagelum(rank 2): https://pastebin.com/8Jy5KUdp
-Predagelum(rank 1): https://pastebin.com/UkSpcqd1
-Predagelum(rank 0): https://pastebin.com/DMfNxZzm
-Amphidator: https://pastebin.com/j5Nu362K
-Alpha Cliviast: https://pastebin.com/B3du8PZG
-Hirondeleor: https://pastebin.com/1B6sTufr