I used to own and trade shares but found betting on the index values themselves was much more reliable through lower volatility. I also found I disbelieve the diversity myth as told by salesmen.
The object of a diverse portfolio is to smooth out the variations in asset price performance over the holding period. This is justifiable when the majority of assets in this class are rising, in order that the risers more than outweigh the losers plus the flatliners. So you could say you should invest in all the member shares in the index but in practice that means a representative sample, or even an optimised sample.
But what about in periods of bear market conditions? Its impossible to say that your representative or optimised sample will be less damaged than the rest of the index. They might fall only as badly but they might fall worse, and there isn't a way of knowing. The risk is actually skewed to the downside because most private retail share traders buy in bull conditions, with no TA visibility of how the same share performs under bear conditions (even if these were standard). Add to which, if you have to sell holdings in 10 stocks in a crisis, the costs incurred will be greater than simply closing a long on the index value.
So your efforts to reduce the risk of lower profit might well have led to an increased risk of greater loss and you will be unaware until after it has happened.