You might want to take a little bit of caution with this advice of putting the spare money in investment, don't put everything in an investment, especially with a volatile one like bitcoin. Take it easy in investing because there is a lot of unforeseen events that may need a lot of money and you put it all in investment which is going to leave you with no choice but to get it out prematurely. For me, crypto is a really good investment and fiat is a useful tool that I can use to buy services and products.