I have a friend who's self-studied up to a pretty good hourly rate within ~2 years. The main thing he does is tons and tons of time studying solver outputs.
In this particular case I've just directly looked at the friend's pokertracker data, which includes rake values and then you just add rakeback to get the total. It's harder to cope oneself in an online setting where there's just a precise recording of all results.
I think you can safely assume that unless someone is part of the absolute poker elite, they are unlikely to be earning anything near the median software engineering income. It’s not impossible, but it would require access to some unusually soft high stakes games, and while such games do exist, they are typically invitation only.
I was a complete blackjack noob until a few months ago when I got interested in my states online casinos.
I would check literally every position (even obvious ones) to see the expected value for each decision and quite quickly built up an intuition when I was playing without the aid (the live dealer games sometimes move too fast to input and get results back in time).
Now its like second nature and only in rare situations do I double check my judgement.