I know it's not the real deal, but at least for some releave to the macOS drop by RAZER, one can get some functionality back on Big Sur with other third party software.
So, for the buttons you can use SteerMouse: http://plentycom.jp/en/steermouse/
For acceleration and smooth scrolling I use Smooze. Both have trial versions and do not cost a fortune.
Smooze can also set buttons to system actions, but it didn't detect the ones by the Naga. I wrote to the developer, and wait for an answer. Will keep you posted in case there is interest.
However, to give you a quick impression how it looks like.
Here you can anaylze and detect everything you need.
Here you can edit the buttons:
Maybe it can lower some frustration about the canceled macOS support. If it even works for my old 2011 Naga, it should also work for newer ones I think. You can try..
cheers