TLDR; Try disabling Time Machine or removing faulty Time Machine targets. After working on this for awhile, I think I've figure out what's causing the issue that I'm seeing with umount and Safari. Each time that umount is stuck and Safari is non-responsive, Time Machine was failing to access a disk over the network. I've seen it both with a Synology Time Machine share and a Mac OS Server Time Machine share. Removing the faulty Time Machine target or disabling Time Machine altogether appears to result in the issue no longer happening. I hope that this helps at least some of you.