Professionals trade with low leverage low return, investment security comes first, end of. Retail Amateurs and mid range traders leverage and strive for returns. I wonder who is right...😎
Your monthly return does not necessarily depend on your amount of risk. It depends on the overall quality of your strategy, your skill level and on many factors like quality of your signals (win-probability), frequency of trades, optimal placement of stops etc.
You can have low risk per trade and high investment security and still achieve relatively high returns.
Unfortunately most people have a wrong way of thinking and tend to restrict themselfes in their abilities and in what they are able to achieve.
I believe, that is why we got the famous 99% poor folks versus only 1% super rich in the world.
The OP's question whether 15% / month is too much for a 100k account is a stupid question actually and any simple answer 'yes' or 'no' is stupid as well, if no further information is given about the skills of the trader and the profitability of his strategy.