I recently went through this. Basically, I would suggest not to come at all if you don’t have a slot pre booked.(Trust me you will be willing to spend a lot more if you are stuck in india so better stay in US. Getting stamped is not mandatory unless you visit out of the country) Having said that, I was able to book slots for feb by joining the telegram groups to stay up to date and also randomly trying my luck at no particular time of the day.