When investing, it's best to do it in lots of coins if you want to be successful in the crypto world. BTC is a favorite coin and safe to hold, but many altcoins make bigger profits than bitcoin. Everyone has their own analysis and strategy. Good luck
Diversification of investment is indeed essential but diversifying by jam-packing your wallet with lots of shitcoins won't increase your chances of profitability at all, it will on the contrary create room for the possibility of more losses.
Like you've rightly stated, bitcoin is the safest and most profitable crypto asset to HODL, and if you ever feel the need to diversify your investments, you could consider buying other assets, such as gold, silver, even real estate assets as they're also very safe and have the potential for future growth, rather than gambling with your money by filling your wallet with ticking time bomb tokens that could be dumped anytime along with your money.