It sounds like you are doing well to me btw, like you say theres perhaps just a few subtleties missing I think.
Plenty to chew threw from last post but small additional point is check what your acceptance is in the Monte Carlo, I am taking a complete guess that you are running indicators and backtesting date tests and that your acceptance criteria for the Monte Carlo is set to profit factor > 1??
If this is the case for example, I expect you might get a similar amount of strategies passing if you reduce the 90% to 80% and increase the profit factor to 1.1 or even 1.2 which may be worth a try, just a side suggestion/food for thought comment though :-)
Also my approach is generally similar and I have mixed results so I don’t claim to have everything figured out at all (but the harder I work the more things improve). I have been at it a while though and certainly tried and pondered lots for sure though so hopefully some of what I have said helps give you some ideas to move forward :-)