It's not really a bad decision to buy Bitcoin now and store it, depending on the duration you choose to hold it for. Since you are working and want to invest in Bitcoin monthly, you can make a budget from your monthly earnings, and once you have some money left, you can invest in Bitcoin. But also don't forget that it's always necessary to keep some fiat so that, during some emergencies, you don't need to sell off your bitcoin so quickly. Sometimes emergencies do arise, and if selling your Bitcoin is the only option left, you might not really be in a good position to sell, and sometimes you may be selling at a loss.
Let's say you receive $400 every month for your salary, you can decide to invest $20–$50 into Bitcoin investment. Just do it in such a way that it doesn't affect you.
In this case, you should not rush to invest, if you can save only $ 50 per month, then the best solution is to accumulate a reserve in fiat so that it becomes your support in case of need and you would not have to think about what you have to sell bitcoin at a loss if you need money.
When there is enough money only for the most necessary things, then it is worth thinking not about investments, but about creating comfort and trying to increase your income, otherwise nothing will change. And when finances allow you to create a reserve, then you can think about further investments.