Personally (given the name of the tool) I recommend something turtle themed.
https://i.imgur.com/kJiaHwM.png
Seeing that icon, I would assume the quality of app I'm getting is not the quality I expect for $30.
Per Apple's documentation, you need icon sizes up to 1024x1024. This represents a 512x512 size at 2x DPI.
https://developer.apple.com/design/human-interface-guideline...
At the very least for the short term you could hire someone at an online freelancing site to trace the existing icon into vectors and make larger versions of it.