You’re in the sweet spot for a huge drop. A combination of water retention as well as fat loss - think of it as the low-hanging fruit. It’ll decrease and taper to a smaller amount but stick with it and you’ll enjoy the benefits.
Man when I spent 2 weeks (same drive thru diet prior) not eating refined sugar, potatoes and soda, I also lost like 15 lbs right away and was peeing constantly. Definitely water so when it slows down don’t get discouraged, you’re about to start the switch between losing fat and gaining muscle, you’ll go anywhere from 3 days to a week not seeing nearly as much weight loss!
When you say “switching to a keto lifestyle”, what timelines do you mean? As far as I know there is fairly solid research that keto isn’t healthy long term. Don’t get me wrong - I am fasting / keto right now and plan to do so for 6 months to a year. I am just under the impression that it is a tool to utilize short term to lose fat and should not be viewed as a long term lifestyle.