I agree with you that we should diversify it means not just focusing on mainstream currencies such as bitcoin, but we can buy something more risky as low market cap coins can grow by 10 times and more very quickly. If you are not fond of risks, you all the same can find altcoin to yourself that has good fundamentals. For example, ADA, DOT, Matic.
The irony is that as btc is at its btc likewise solid altcoins are also at there ATH or standing at there peak value. We need to wait to invest in top altcoins or do some rigorous research and find coins that are undervalue even in current bull run. But top time is when market is bearish, sadly most of us are running away from market when it's the actual time to buy.