It's hard to keep anything going only during weekends as you spend 90% of your energy just trying to find momentum.
Re "But after one day of programming for my job, I am exhausted and I cannot extract any brain-juice any more. And if I try to work during the week-ends, I can't rewind enough for the next week" -- I had exactly this problem too. It depends a lot on your other commitments, but after years of failing to do anything productive in evenings and weekends (evenings, too tired; weekends, too easy to procrastinate), I partially solved this by spending 30-60 minutes every morning on side projects. Waking up earlier was hard, but I managed to get into the habit of spending some time in a cafe on my way to work. The change in environment (not home) and the very limited time (often I only have one well-defined goal for a morning session) makes me super productive.
I still battle with consistency, but I create a lot during these times. One thing I'm working on is a guide on how to start your own company in 30 minutes a day. It's still at idea stage, but there's a (currently partially broken) website slowly being pieced together[0]