Since these payments are considered taxable income, I am not sure why they would think they could avoid Google's rules just because they are a "small one-man shop".
The simple solution here is to add in-app purchases like every other "small one-man shop" and not ask for noncompetitive advantages and accept Google's 15% cut [0].
[0] - https://android-developers.googleblog.com/2021/03/boosting-d...