I would Say the first thing to do before investing into bitcoin or any cryptocurrency or even any investment outside this is to accept that it can go down and refuse to climb up again and that’s why we call it a risky investment: I know most people will say bitcoin isn’t like that and it is true but having that mentality will make you invest an amount that you can afford to lose
Agree with your point about investing using money that we can afford to lose, it's literally the reason why so many whales are making it big every bullrun cycle with bitcoin because they can afford to wait until the price gets better.
it's like that study about the deceased investor having better investment return than the alive ones because the world literally favour holder of good commodity or asset and will give them big return as the time goes, because if we can observe the inflation it's always increasing every year, meaning the money we invested in an anti inflationary asset alone already gives us profit. now if we invest into an asset that's both inflationary and fundamentally good, the only way for our investment portfolio is only to go up.