If you're talking about capital productivity, alternative investments, and what is possible, I suppose the most important number would be return on invested capital (ROIC), possible risk-adjusted, over time. I agree that it helps to know what is reasonable, as the sports analogy shows. But it is also important to keep in mind that trading involves a lot of chance, probably much more than in sports. Determining how much of a trader's performance is due to chance and how much to skill is very difficult. If it were easy, money managers would have much better records than they do. "Average trade" is a good metric for comparing two similar systems, such as a system with and without optimization. But it is useless for evaluating dissimilar systems. Same with "total profit", the newbies' favorite.
There is no opportunity cost in trading. Since you can make sooo much money trading, the consideration to menial employment is not a factor. The excess rfr is also not applicable since it is too low. About the 3 month or six month. I think you're more afraid of whether being profitable long term is possible or not, but even still the enormous returns that are possible negates this and thus the sharpe ratio will tell you if it is worth it or not, and where the rfr is so low all return is excess to a point.