I think the best thing I did when I first learned about Bitcoin, was to take up a second job to fund my Bitcoin purchases. The income from that second job was not something significant, but it was enough to kickstart my Bitcoin journey.
That small amount grew into something very significant in about 8 years, with the increase in the value of those coins. We are talking about 800%+ during that period, something I would have never been able to do with any other investment option.
Do something extra, because that little bit of effort will reward you in the future.

You are right. Relying on your single job alone won't be sustainable in the long run. You really have to find a side hustle that will give you extra income that will cater your investment funds. And when you come to manage it properly, it will definitely come into fruition and give you a lot of earning opportunities. Sometimes, it takes encouragement and patience for you to be able to do that. Otherwise, you won't experience investing in bitcoin successfully and profitably.