I would put Guardians as a relatively high priority.
Realize that you have a choice where to use Gems, Challenge coins, and Epic Hero tokens, while Costume keys and both Atlantis & Valhalla coins bring no choice on where to use, just when to use. That’s often about what is featured, but also when you need them.
Epic Troop Tokens & regular Summon Tokens, but those are a when to use only based on when you need troops, feeders, or to do summons for some reason, there’s no real advantage to timing based on featured heroes/troops.
So the key is really prioritizing all three of those resources, and you want to figure out Challenge coins & EHTs first, as they are limited. The goal is to have a plan. Your plan will likely not be my plan, or anyone else’s plan, but it should make sense generally, and specifically for you.
I prioritized Guardians to get better at Titans, and to get better loot, and it’s worked. I’m rarely scoring an A+ grade in my alliance, due to people with deeper pockets and better summoning luck - there’s somebody where I have literally 1 hero they don’t, and they get HOTM on average every other month, it seems. I get A grades almost all the time, though.
I missed the point about hero space. I spent a lot of gems there, as I’m not a spender, and wanted to be able to efficiently level up heroes with 10 on-color feeders at a time, plus hold trainers for use on 5* heroes late in process to save food. I have what I think most would think is too much room: 192.