Not sure how you come up with $550 unless you are talking another country? It's $400 in the US.
Regardless, it really is a matter of personal preference. Spend the money if you got it or invest in a lot more external storage. If you are worried about the 'look' of a small SSD drive then for the old iMac you could get a tray that extended like a shelf off the back support so that it is completely out of your field of view in the front. I actually have two hard drives on mine. Sure if you are looking at the back you see it, but even then it has to look better than on your desk? No doubt there will be something similar for the new iMac.
another point is, I always have a time machine back up attached to my computers, NOT having an external drive of some form is never an option. So once you bite that bullet... why not just get 1 tb internal and call it a day.
Twelve South'S Backpack Is A Smart Steel Shelf That Hangs On The Back Of The L-Shaped Stand Of Your Imac Or Apple Display. The Sleek Metal Design Is Perfect For Storing Things You Want Out Of Sight, But Not Out Of Reach, Like Hard Drives And Hubs. Not Compatible With Imac M1
www.amazon.com