Have you successfully followed this plan? It seems to have a couple of holes in it.
I tried #2 at 3 separate jobs over 10 years, to no avail. Best I got was "does it have to be on the books?" i.e. I could call in sick or take random afternoons off and my manager would look the other way. If you are in demand, companies want you to work more, not less.
I was, however, able to negotiate 80% time at my last job, because I made it a condition of my employment before I started working. But that was a hard sell too. In particular they wanted to make sure I was not going to be moonlighting with my extra time. I don't agree with that attitude (my personal projects certainly take a lot of time and energy, even though I'm not making any money with them), but that's how the market works, even for a software engineer.