I own the S21 Ultra myself MikeB and straight off let's get to the issue of that 100x zoom. It is all an advertising gimmick to capture potential buyers. Which is sad as the 10x optical zoom lens on the phone is brilliant in my eyes and they should focus on this. Forget about zooming in to anywhere near 100 times for any shot. The 30x preset is for moon shots only in my experience as it is too contrasty in well lit conditions. Simply taking a picture at 10x optical zoom and then cropping in on this image gives the best 30x image. And indeed an image that no other phone can beat at this magnification. I marvel at the results of zooming in using this method.
I have never had a phone with Sports Mode MikeB but I must say that in good light I find the main camera lightning fast, capturing moving cars alloy wheels at a dead stop in pictures but when the light drops the lens speed and processing skills drop off noticeably to the point that any action shot indoors will come out a little blurred or even very blurred sometimes. I understand how you must feel if you enjoyed taking pictures of your dogs using this mode on your S10. I assume this mode worked well indoors and gave you better results outdoors also?. I have not used Pro Mode on my 21 U but maybe someone on this forum can give you advice on how to speed things up camera wise for such action shots?.
I love my camera on my S21 Ultra as it does everything I need except shoot good low light video to this point in time, which doesn't bother me as I use it outdoors nearly all the time. I have taken many pictures on my home island Anglesey and have submitted many to Google maps over the last 3 weeks or so which have all been published and I am pleased with the results as are Google Maps and the 5,000 people and climbing who have viewed them so far. For me, owning the S21 Ultra is liking having a quality DSLR or mirrorless camera in my pocket whenever I need it. I feel for you MikeB as the phone is disappointing you from the off, and if I was you in your situation I would be disappointed too as there is nothing as deflating as dashed first impressions. I hope you can move on and discover what a brilliant camera your phone truly has or just move on to another phone that gives you what you want. There is nothing worse than feeling angry or frustrated as I have discovered but it helps to vent things for sure. Good luck MikeB and I hope you do find what you want and are happy with one way or the other. 👍👍