If you strictly follow the wiki post #1 in this Forum (Opencore on the MacPRO) you will learn how you need having Mojave as base (the last native OS for cMpros) in order to install Catalina
It is still under investigation one stable version and guide for installing BigSur in OpenCore with Mojave as a basic OS. It seems it can be a question of weeks.
But I (and many others) have installed a fresh BigSur ( and recently updated to 11.1) using the same process explained in post #1 for Catalina. Hardware acceleration (just for encoding 4K video and watching Netflix in Safari) is not working at the moment. Some people (not me) are experiencing USB and Bluetooth issues.
Good Luck!
Being honest here, at this moment, Big Sur is not usable for a lot, if not most, people with MP5,1. If you use Bluetooth for you keyboard/mouse or your workflow depends on USB interfaces, stick to Catalina for now and avoid a lot of grief.