I'm in exactly the same situation as you. I was eagerly awaiting the S21 but left feeling a bit disappointed. The S21/S21+ I actually feel is somewhat of a downgrade and the Ultra is going to increase my monthly cost by over 30% and I'm not overly sure other than the camera what I'm gaining hugely as my S10+ has been updated with the Android 11 update. I'm not that bothered about the S Pen, only use the camera for general snapping (I can barely tell you what all the other functions do
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) and battery life of the S10+ is enough for me to get a full day out of it.
Debated iPhone 12 Pro Max but against the S21 Ultra has the same price issue and isn't as good - Pixel feels like a downgrade in comparison as well although has the raw Android experience without the bloat.
I'm currently hanging fire and have just kept my S10+ on a cheap SIM only deal for now which is out of contract so can jump at any point. Debating the Sony Xperia 1 III when that comes out or maybe might splash out on the Galaxy Fold III when it comes as read on here it will have AMD GPU in it. To me none of what's on the market as things stand overly offers enough of an upgrade for what I use it for so blowing over £1K is crazy until I find "the one". I appreciate new devices are all realistically going to be over this price but for me they need to do more before I part with the dosh!