Yes, I think that a fast day after doing 16:8 can be tough if you are using 16:8 to cut down on calories because you're already in a calorie deficit situation when you start! As jmousey says, Monday is a good day to do it if you tend to be more indulgent at weekends. The other thing is that you don't have to go straight down to 500 cal if you find it daunting...you could start with having an 800 day, then next time go down to 700, then 600 etc.
However, as 16:8 is working well (2kg in 3 weeks really is awesome) you don't really need to add in a full fast day if you don't want to, so another alternative is to think of the fasting period as being the important thing so rather than sticking to a set calorie count, consider having one or two days a week with a much shorter eating window, so instead of 16:8 do a 20:4 or a 22:2, and don't worry about the calories.
There is no 'one size fits all' with intermittent fasting...you have to try different ideas and see what suits you bearing in mind that the basic premise is to extend the time when you are not eating and in so doing cut down on calorie intake.
Good luck!
However, as 16:8 is working well (2kg in 3 weeks really is awesome) you don't really need to add in a full fast day if you don't want to, so another alternative is to think of the fasting period as being the important thing so rather than sticking to a set calorie count, consider having one or two days a week with a much shorter eating window, so instead of 16:8 do a 20:4 or a 22:2, and don't worry about the calories.
There is no 'one size fits all' with intermittent fasting...you have to try different ideas and see what suits you bearing in mind that the basic premise is to extend the time when you are not eating and in so doing cut down on calorie intake.
Good luck!