You shouldn’t do long fasts if you haven’t before and are on medication -> consult with doctor
In general, you’ll be better off with something that you can do consistently over long term (basically forever). That’s why I’d suggest intermittent fasting, omad or ADF over trying for 20 day straight fast.
Build up to a long fast. Don’t just stop eating for 20 days (that is an extreme fast), and you most likely will fail, and do harm to your body.
I did OMAD until that was easy. Then I did ADF. Then I did a 3 day. Now I am finishing a 5 day and struggling to make it over the finish line. I will do another 5 day next time, then perhaps jump to 7.
Losing weight is a marathon not a sprit.