You can buy anytime if you are planning to hold it for a long period. Because as you can see, Bitcoin and other coins are still in the dip compared to its price way back 2017. It is cheap right now.
I also believe that Bitcoin will rise higher and higher and altcoins will continue to fall, I now buy very little, but still can not resist the prices that are now, I hope I will pay back my investments
When Bitcoin rise, altcoins also rise because its value is based on Bitcoin's price. So if Bitcoin hits $100,000, the equivalent price of other coins will be converted based on the current Bitcoin's price.