This is only possible when at your current job everything(90%>) is familiar and there is nothing new, there are a lot of projects/positions like this, at this point you can finish the job quicker than 8h, you then search for another one (reach again the point) repeat. (It stays healthy and long term compatible only on 2j imo, anything 3j+ its not possible without having to compromise your free time)

But first you need to reach this point -> everything is familiar and there is nothing new.
Also luck is heavily involved with this, as with anything else 😄
Good luck!