I have the S-Pen with the S21 and it does give a lot of functions, so it is not money wasted. The only thing it doesn't support at the moment I believe, is the "air gestures" and remote control bluetooth function.
In the meantime, you may be able to get a second hand pen from previous generations of note, or galaxy tab, which should also work with the S21 ultra.
I bought the S-Pen with the case... and I really like the size as it is more manageable than the smaller Note stylus and better for jotting down longer notes during a meeting.
My thoughts on a release date for the S-pen Pro... are that the entire electronics world is struggling right now with parts availability, silicon shortages, and generally speaking this is affecting smartphones, PS5, Xbox, Graphics cards and a whole load of other components and electronics.
Therefore, I would not be surprised if release dates are missed, items do not get into the public domain at a date that they were expecting to. So I would wait with baited breath...
Personally I will probably buy the S-Pen Pro when it is available, but will wait until a opportune moment when there is a members discount voucher or online sale... i don't wish to pay £50+ for a stylus if it can be helped ! 
Smiley.
found ... needed some extra software and a update
