At this time we strongly advise you NOT update to Big Sur or MainStage 3.5.
Our advise is based on experience with a large number of users encountering a wide variety of hardware compatibility related issues every year when a new MacOS is released.
Big Sur in particular looks to be a very big update under the hood so we anticipate a high likelihood of initial hardware (and also software plugin) compatibility issues in the weeks after release.
We recommend that you look to the manufacturers of the hardware you rely on to see them give the all clear for Big Sur before you update.
What about Sunday Keys and Sunday Sounds' products specifically?
Since our products largely work within existing software like Ableton Live, MainStage 3, and Omnisphere it's best to look to the manufacturers of those softwares before updating.
Ableton and Spectrasonics have both advised their users hold off on updating.
Since Apple is the developer of MainStage 3 it's highly unlikely that Apple will encourage their own users not to update, but in our experience MainStage is not always immune to initial OS related bugs.
For this reason again, we recommend you wait to update to Big Sur.
Specific to Sunday Keys and all of our products, we'll also be working with the release build of this new OS to make sure there are no issues introduced by Big Sur.
In the meantime, if it ain't broke, don't fix it!
If you have a compelling reason to update and just can't wait, make sure to do a full Time Machine backup of your system before doing so, as that will allow you to revert back to your prior OS if you run into any game-breaking issues.
As always, we strongly advise against using Apples "Auto Update" feature.
We will communicate any issues that early adopters run into as we are made aware of them, but again we recommend that you keep checking with the manufacturers of the software you use to see when they give the all-clear. We will also be sure to let the community know should we become aware of any prevalent issues.