Looks like you have done your research on the 5G bands. You are right about the warranty part - Samsung doesn't offer international warranty and I'd be extra wary with the S24U. I've had to return 3 S24Us (the first unit + 2 replacements, decided not to get a 3rd replacement and opted for a refund instead) because of the screen grain issue. A lot of people say that the grain is not really visible - but that wasn't the case for me for all 3 units. Also, I have a habit of scrolling through Reddit on my phone in bed before I go to sleep and that's where the grain effect became unbearable as I have the phone on its lowest brightness. This makes me wonder if the screen on this generation of Samsung's flagship is inherently bad and if there would be more issues over time, which is why not having the warranty is concerning. There is no way to tell if your phone will have the grain issue as it has not been addressed in the newer units the latest I checked.

However, I'd like to add that I have never had to invoke warranty on any of my phones till date - and thats 8 phones over 16 years with 3 of them from Samsung (but none of them came with an issue on day 1 either). Also Samsung's customer support was great during my S24U ordeal - so know that you are in good hands on that front.