I wonder if that’s it for sketch up make then in Mac OS. I don’t like the browser version of sketch up but if I upgrade to big sur I may have no choice.
One other solution is to set up a virtual machine. I have a 2012 MacBook (running High Sierra) and use a very old drafting package which runs on Windows XP. I used VirtualBox free version to set up the virtual machine with XP and the drafting software in it and no connection to the internet so no danger of viruses. The design files all sit in the Mac part of the machine so printing, emailing files, backups etc are all done using the Mac operating system. All works seamlessly, XP opens in its own window, open the drafting package and one of the “disc drives” for file location is a link to a folder on the Mac OS. For printing I produce Pdf files which are saved on the Mac OS. I then print in the normal way and I can print exactly to scale.
You could do something similar, upgrade to Big Sur and have a virtual machine running either an old version of the Mac operating system or an old windows operating system. Run sketch up in this and have your design files in the Big Sur part of the machine. You may need to give some thought to printing. I am not familiar with sketch up and what sort of print file it produces.
i cannot answer detailed questions about how I set up Virtual Box. I did it just the once and googled how to do it. It took a few hours but now cannot remember exactly how I did it. It went fairly easily with no major issues.