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, the items take 2 times less time to collect, meaning you get twice as many items during those 2 days, which means you effectively get 4 days worth of items in that time, which can also be translated to 2 extra days. Note that recently island boosts became less common so scroll

-To find the strategy that suits you, think about how many times will you collect an entire pool of items in the 11 days of the island, and multiply that number with 8 hours to get the amount of total time you want on your strategy. Note that if the number of hours is divisible by 8 on all collections, I put the number of collections as a reference and not the number of hours.

-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, the strategy tells you the number of gems to spend on that quest. Note that quests which only require 1 breeding 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 if there is no island boost(or if the number of hours is divisible with 8, then the number of collections), which is mainly helpful if you divide it by 8 hours to see how many times you should collect the entire pool of items after you get your first collection. Note that dividing by 8 also works if you're planning for an island boost; just make sure to collect every 4 hours instead of every 8 hours.
If you're going by every hour collection, you might see a remainder when dividing by 8(usually 4, when you get 3 gold and 2 food items without waste); that means that it's optimal to collect an incomplete pool and then gem.

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

Anyway, here are the strategies(collection of what you can every hour):
Sergent Hull Head: /pastebin.com/qTchGCZx
Sergent Hull Head if first gemmed after 4 hours: /pastebin.com/znmehX15
Sergent Hull Head if first gemmed after 8 hours: /pastebin.com/muiWMRJC
Scrap Warrior: /pastebin.com/1L9NQVJJ
Scrap Warrior if first gemmed after 4 hours: /pastebin.com/CVbB5s4A
Scrap Warrior if first gemmed after 8 hours: /pastebin.com/c7WDRBs3
The Sentinel: /pastebin.com/SQL2R1bz

And here are for collection every 8 hours:
Sergent Hull Head: /pastebin.com/QnhHrXLw
Sergent Hull Head if first gemmed after 8 hours: /pastebin.com/jLcTS8Z1
Scrap Warrior: /pastebin.com/qpcVGhw7
Scrap Warrior if first gemmed after 8 hours: /pastebin.com/UPMuDj6u
The Sentinel: /pastebin.com/Y26p931R
(note: the "if first gemmed after 4/8 hours" strategies are there because, sometimes, it's optimal to skip the first quest right away, but some people didn't do that because this post came out late.)