This Amazon Buyer review "So much promise and anticipation only to be let down, not without advancements at a cost" has an interesting perspective:
https://www.amazon.com/Samsung-Elec...998UZKEXAA/dp/B08P54F6HB?th=1#customerReviews
IMHO, only reason to upgrade from S20+/Ultra or Note 20 Ultra to S21 Ultra is for the 10x telephoto (if you feel you could really use it.) The glass, display and battery life improvements are relatively minor on the S21 Ultra over those previous three models. And you end up losing SD card slot and getting a bulkier and heavier phone (for some that's good as "big is better more flashier" - for the rest of us, not.)
If you read the reviews for the S21 Ultra on the Amazon listing page, the first most popular review really says a lot. Notice he doesn't go into camera details. As the bottom line is, unless you absolutely are starving for a 10X optical zoom, the S21 Ultra is a waste of money coming from the S20 Plus/Ultra. You'd be better off getting a Note 20 Ultra that has 5X optical zoom, SD card slot and big 6.9" screen rather than getting the S21 Ultra.
Some excerpts from the Amazon Review:
"I found myself missing the S20 Plus, and more the S10 Plus, even with the new upgrades. I've lost enthusiasm and confidence in Samsung. The S21 Ultra's weight and size is not for me, I found myself with hand fatigue and not really enjoying using the S21 Ultra. The Note 20 Plus is actually the better phone than the S21 Ultra if you want productivity and comfort."
"If you still own the S10 Plus, keep it, if you own the S20 Plus, keep it. The S21 Ultra, in my humble opinion is Samsung reducing itself to become the villain. It's harsh and it's my opinion."