You must travel through time and back to 8 years ago. And with regard to the OP, It is difficult to know what is the best and appropriate time to buy a large proportion of Bitcoin, because the events change over time, and there are always unexpected surprises in the market, for example, months ago, when the price was huge ($ 18K), all people thought that this currency would jump by a large percentage and will become more than $ 30K. But unfortunately, there was a reversal of expectations and the price fell to $ 6000.

As a matter of fact,  the opportunities must always be exploited in a smart way, You must have different strategies for each new event, you should not buy in a random way, because that would lead to many financial losses. So there is no best time to buy due to your choices may will be wrong,  and  you can even  behave well in a difficult time, just you must have a futuristic vision and realistic.