You’ve pretty much given all the information you need in your post. You’ll be 6400 calories below maintenance every week and a kilo of fat is 7700 calories, so it should take about 14.5 weeks to lose 12 kg. There’s lots of assumptions in there that might not be accurate, but I’d give that as the best guesstimate based on the information you’ve provided.
Also, you’re really doing something like 60 hour fasts every week unless you’re eating right before midnight on Friday and right after midnight on Sunday.
I’m 5’6 165lbs. Been working out for decades and was 190 at my heaviest with the most muscle. I’ve always had a gut and never even came close to having abs or the pelvis lines. Was always concerned about sufficient protein so I could stack on max muscle. But then I was always a kinda fat dude with big arms. Could never cut the rest of the fat properly.
I’m trying a different method now. Veering away from the power lifter style body and want to shredded. Cut down to goal weight(155) and then stack lean muscle back on with a very precise refeeding diet. Lean protein and veggies, minimal carbs. Down from 175 to 165 since Jan with rolling fasts. 48s, 96, 72…etc. I’m currently 40 hours into my fast today and have been lifting last few days and feel great. I would think your goals would be achievable if youre really careful with what you’re eating when you’re feeding. If youre.not getting the results you want extend your fasts.
Good luck!