As others have said, don't be so hard on yourself: that you're teaching yourself useful stuff is the opposite of zero productivity. In fact, you're probably learning faster than if you were doing it while employed.
My best suggestion is that your local library will likely allow you to download an app such as Libby that will allow you to listen to audiobooks for free. During my downtime, nothing has brought me more respite from my depression than listening to - no joke - about 200 books. I alternate between sci-fi and non-fiction. I never would have imagined myself a listener, but I was very wrong.
Just put your routine in a FOR-loop and grind for increased STRength and STAmina stats lol
Active VR games are a legitimate cardio workout. Sites like https://www.vrfitnessinsider.com/reviews/ have real data if you don't believe me.
As for duration, I've seen people (including myself) happily play Beat Saber for hours who don't last 10 miserable minutes on a treadmill.
Off the top of my head: Certain oils are pertinent -- some should be avoided, others sought out -- and iron deficiency can cause depression.