Buying only Bitcoin or at least 90% Bitcoin.
I do agree that BTC must be your primary target of investment in crypto. IMO 60 to 70% of your capital should go in BTC and rest of capital for strong alts (top 10 alts at CMC) and short term trading. OP point is also valid that when BTC goes down every coin goes down with it. We have to accept that fact, the logic of equity trading don't fit here.
People don't get this strategy is still supreme than diversifying a lot. It's okay if you want to diversify but be sure that you've got bitcoin as the king of your crypto portfolio.
More than half the percentage of your total assets, that's already good. And it will depend on the likes of the crypto that you want to hold because if you diversify a lot, it's not that good too. So always weigh and balance not just your portfolio but as well as your decisions.