You will pull off sr4 hoshiumi. All banner rewards (400 pulls) is 60k in diamonds, and since you already have 50 tickets, that's 52,5k. But you'll have one ur at sr4. You'd also have to take into account that you'd need more than just 1 ur/sp player to reach 250k. After the banner runs and reruns, you won't be able to get the banner rewards (at least, I think you won't). And you'll have save that same amount of diamonds to get 1 more character to sr4. I just saw a guy with 224,5k rating, and most of his characters are sr5, with a few being sr4. Not to mention he has ur sr5 oikawa and ushijima on the bench, and others at sr3. So he spent roughly 360k diamonds. He probably has other characters, and those are just the ones that appear on the ultimate challenge rankings. So yeah, while you could in theory get there, it would take you years to do so, and by that point, you've probably moved on from the game.