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 !
We now know the S Pen pro which will work with multiple devices available from 27 August but can pre order. Obviously with the bluetooth connectivity and the possibility of additional premium features in the future. However the Air Gestures require a software upgrade. A bit pricier than I expected though as it retails at £99, have not seen it listed in othee stores yet. https://www.techradar.com/uk/news/samsung-galaxy-s21-ultra-works-with-an-s-pen-pro-but-what-does-it-...
A quick guide for the S Pen pro here, it needs to be paired to your Device . Obviously look out for updates. https://www.samsung.com//SPen Pro