At first my limit was on the temperature, but now my phone temp is only getting to 31 degrees Celsius. My CPU is only reaching 70%. I don't know what's limiting it at this point. And ideas?

Like laptops, phones generally have heavily restrictive default settings for power usage. When on battery, the settings are even more restrictive than when plugged in.
In the phone's performance section, check to see what the settings are for power and thermal limits, for when on battery and when plugged in. And depending on the charger, full CPU performance may not be possible while charging- only once fully charged, due to the limited output of the charger.

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Grant
Darwin NT.

Unless you possibly want to blow up or have your phone catch on fire NEVER USE ALL CORES.
Lithium batteries can and do explode from overheating!
Generally you should only use 50% of the available phones cores for BOINC to prevent overheating and battery problems.

Below is an article that explains the different types of cores a Smartphone has and why you can't really get use of all the cores.
Quad-core vs octa-core: does Android and apps use all the cores?