The main thing I had left to try was a clean install, but since I had done one of those when I installed macOS Eventually, I found some hints that a folder containing a very large number of files could cause Dropbox, and thus the Finder, to choke. However, I have some folders that are legitimately large and not just full of junk. I hope the solution sticks this time. You probably have an idea of which folders contain vast numbers of files, but if not, DaisyDisk is a handy tool to find them. I had observed a phenomenon similar to yours : I had the Finder freezing from time to time, and sometimes I had to wait for 10 minutes to get back control sometimes I had to restart the Mac.
In all these cases, fans were at the maximum speed during the hang-up ;-. I had the Downloads folder full of files as well : 26 thousand items for 21 Gb! So I turned off the DropBox Finder integration two days ago and, presto, no more hang. Thank you very much! Update as promised : all hangs I experienced for more than 6 months are gone!
Thanks a lot! Dropbox does have it limitations. Random freezes have been driving me crazy for months. No way to tell.