Hmmmm, you are more in line with the conventional thinking, to not hold these long term. Volatility decay is the killer.

I've read a number of papers that did a deep(er) delve into the math. Two points come through:
1) Optimal leverage is about 2X.
2) DCA is better than lump-sum.

And, of course, don't use leverage in a bear market. :-) ;-)

I think another point would be the same as what people have said about options investing -- do not compound your returns. Decide your position size and take the profits off the table.

Now I've got to go back and take another close look at my timing signals.

I bought 3 tranches of TQQQ, they are currently up 46%, 73%, and 95%. Leverage is 2.5X, 3.2X and 3.5X.
At the time I bought #2, #1 had a loss of 15%, whereas QQQ was down 3.5%.

The idea of 15% gain or 21 days hold is interesting, I never thought of that.