Studies by Dweck, famous for Growth Mindset, and others point to "ego depletion" simply being a reflection of people's beliefs (My willpower is limited) as opposed to an actual resource that gets depleted.

https://www.researchgate.net/publication/46579000_Ego_Deplet...

https://digest.bps.org.uk/2018/01/29/another-blow-for-ego-de...