Use market regimes cleverly and invest optimally

Anyone who has inherited, sold their business, won the lottery, or has a larger investment sum due to other circumstances is faced with three fundamental decisions:

1) What should I invest in?
2) Should I invest immediatly or wait for a market slump?
3) Should I invest the entire amount at once or divide it up into instalments?

While the main focus of financial news is on the first question, we focus on the last two questions. Our working hypothesis is that it is almost impossible to find the perfect time to enter the stock markets. Anyone investing for the long term should therefore start quickly, as waiting too long often costs returns. Is this hypothesis correct or does it pay to hold financial reserves in order to buy in the event of a market slump?