There is no ideal or correct time for investment. Every time is appropriate if you want to invest long-term. Whoever bought at 2017 ATH can sell now and his investment will have been lost, but we can say that this time is the best time to achieve the maximum possible profit in the shortest possible time?
In theory, it is correct, but the price may be in the area between 25k to 32k at some point in the beginning of next year before the upward line continues. Therefore, now is an appropriate time to achieve maximum profit, but it is not the best.
Exactly.
There's no need to buy at the deepest dive price of Bitcoin, buying right now is still a good investment since we're still far from the last all time high price. There's no need to hesitate when investing in Bitcoin, even though you bought at the peak price, you only have to wait until it makes another all time high to make profits in your holdings.
It's all about patient and it's important to know what you're doing so you won't get bored or do some panic selling that you will regret at the end.