Not sure what I will do on Thanksgiving although I am sure I will go over my keto carb limit. With regards to the symptoms you mentioned afterwards I can relate what happened to me a few weeks ago. For what it is worth, I am also doing 20:4 IF as well.
The wife and I went out on Saturday for our anniversary and being the classy people we are, we went to one of the nicer AYCE Asian buffets here in the Seattle area. I knew I was going to eat more than I normally do, certainly in the carb/sugar department but I also knew that I would enjoy the foods as an unusual treat. Mind you, as someone who could eat a whole chocolate cake with ice cream and milk or see a box of Capt Crunch as 2 servings at best, I loved my sugar. These days though, I find that while I don’t crave those things, I do enjoy them more when I have them than I ever did before.
Anyways, I ended up treating it as OMAD and ate a lot of things that had carbs in them from the various wraps in the steamed buns, shumai and potstickers to the 2 bowls of ice cream with strawberry and chocolate sauce, cookies, rice crispy treats, chocolate dipped strawberries, creme brulee and cakes. I enjoyed all of it as well as the various meats and such (the bacon wrapped shrimp were a surprise). If I had to make a guess I would say I probably ate around 4000-5000 calories and probably 400 or more carbs easily.
On Sunday we went bowling in the morning and it was about 12:30p when we finished and we went to Sizzler and had the AYCE salad bar and I again treated it as OMAD but had similar fare, probably closer to 3000-3500 calories and again with around 300 or more carbs with the pasta and breading on chicken wings and tenders as well as ice cream, bread pudding, apple crisp, bread and such.
The result? Saturday when i left work (where I weigh myself) I was 243 pounds. Monday morning when I weighed myself I was 249 pounds. Now obviously a lot of that is bloat and increased water weight but none of what I did derailed me. I just went back to my 20:4 IF and eating keto and after working 12 hour shifts all week and 8 on Saturday, I was back down to 243 on Saturday all while eating 2000-2800 calories during the week with zero exercise. I did find that my lower back pain seemed to come back and I think that is similar to when Richard has talked about going off the wagon and having his knee pain come back for a few days because by the end of the week it was fine.
For me it didn’t cause any relapse or hunger pangs or anything like that but I am not everyone else. I have been able to deal with it and maybe in part because while doing 20:4 IF, I do get periods where my body thinks it is hungry during the day and I just manage it. Drink some more water and it goes away.
YMMV of course though.