I haven't found a replacement either. Preview works OK just for adding text markup, but there is no substitute for the Bluebeam Studio interface, or any of the more advanced markup.
I've emailed support again, and will also request that they open-source the Mac version once I get the rejection. 100% sure it's not going to happen, but hey, why not.
Also going to open Revu on my Big Sur Mac as many times a day as I can to get a lot of crash reports to Apple. Shooting for 100 per day. Maybe it gets noticed. Long shot, I know.
The specific error seems to be "Attempt to pop an empty graphics context stack" and I have seen other apps with this problem, so maybe there's something Apple can do. Again, long shot, but it's the only shot for me outside of abandoning Mac.