We should think more about the risk of having all your investments in one single portfolio, such as bitcoin. Bitcoin happens to be the most reliable cryptocurrency for investment but at the same time, Bitcoin is a highly volatile asset. So I will still advise you to diversify your portfolio into multiple sources.
You don't have to diversify because, as you said, bitcoin is the most secure cryptocurrency. But if bitcoin is having a bad time, any other coins will feel even worse. So don't buy the whole deposit at once, but buy in parts. That would be the best strategy. And if the market situation becomes favourable, you can consider buying other cryptocurrencies.