I always use weaker glasses on my computer now (around 1.5 diopters less than my full correction). I just reduce that reduced correction step by step (recommended is .25 diopter reductions, .5 worked for me too though) when it feels right (aka when I can see the screen decently well even with the new weaker glasses). So I started by using -4 glasses at the computer and now I use -2.

To check my current eyesight, I use a printed Snellen chart and the focal calculator https://endmyopia.org/focal-calculator/calc.html

The focal calculator is less accurate, but over mutliple measurements it averages out and adds up to a similar result as the Snellen chart for me.

No, I'm in my early twenties.

Yes, fully corrected, I can even see better than before to be honest. My eye doctor told me last time (before I started endmyopia) that I only have around 80% vision, even with glasses. I noticed that when playing tennis, I had trouble telling if balls were still in the opponents field because I couldn't see the back line properly. Last time I played I noticed how much better I could see it :)