I agree that bitcoin is the priority coin for investing and next is ethereum. I have these two coins as main assets. And I also diversify like you by buying BNB, Solana, ADA, TRX, Matic, Hbar etc. Because I am trying to increase the number of opportunities to get bigger profits from the many coins I buy during bearish seasons.
Not only you, I think most of crypto investors must have Bitcoin and Ethereum on their wallets. Bitcoin and Ethereum are the most recommended coins, they are also the most popular crypto coins. And they becomes the coins that can improve their prices consistently in every bullrun season. That's why people prioritize them to be the main assets.
Yes, some top altcoins are good for diversification. BNB, SOL, HBAR, MATIC, ADA, TRX, XLM, DOT, ARB, and LTC are some options for diversification. However, I suggest to not choosing too many altcoins because it will be harder to monitor their improvement if we are holding too many coins in our wallets.