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As far as I know, every investor needs to have basic knowledge, start with small amounts and stay away from fear or panic to start investing in Bitcoin. You are right discretionary income is a very important consideration when investing in Bitcoin the truth is that with discretionary income, any investor can be free from stress and unnecessary worries, and it becomes easier to buy Bitcoin on a regular basis, perhaps on a weekly basis.
The rate of investing from any investor should be based on the rate and extent of his income. Not everyone can invest weekly except your income comes in weekly manner. If you plan investing on different dates outside your income period, you will find it difficult to hold onto your bitcoin for long or you find it difficult saving up your fiat up to that moment outside your payday. It's better to plan yourself against premature sales especially this period when bitcoin's volatility seems to be high.
By the way, fractional amounts of Bitcoin can be purchased, which should be kept in mind if an investor is panicking before buying because it is possible to buy small shares with a small amount of money with the aim of increasing the Bitcoin savings in the portfolio, so this is a quick estimate that since the price of Bitcoin is now around 63k, at the moment, about fifteen hundred plus satoshis are available for one dollar to enrich the Bitcoin portfolio.
Yes for sure this is one of the beauties of bitcoin investment since it gives you a flexible approach to owning a large stash of bitcoin in the long run because it may be nearly impossible to save up money to buy an entire 1 bitcoin at once but by buying in fractions everyone has the right to own bitcoin both rich and poor.