Some of this sounds an awful lot like a problem I had before.After a while Nuendo froze before being able to fully shut down, and eventually even opening up a project could take a lot of tries and restarts.
First of all I’ve noticed that Steinberg - full stop - is extremely sensitive to drivers to function well. A wee program called Driver Easy swiftly and nicely took care of that problem.
Secondly, and this has haunted me for quite some time, every single issue I had with these things disappeared completely after I back-tracker the logs after an especially frustrating boot, and tried to disable the VST2’s that was loading when the damn thing froze up during initiation. Long story short - I now have a lot of VST2-plugins I will never ever use again, and my system is now rock solid. For some reason though I’ve never had to disable VST3’s, not counting Brainworx before its major update, that simply killed the channels altogether instead, and a few weird small things I’ve got from sales from what turned out to be pretty terrible developers anyway, but none of these things ever prevented a boot up or proper shut down, they only became issues when trying to open the channel for some good ol’ editing .
Most of the fritzy VST2 plugins were either free or pretty cheap ones, but some years ago huge culprits were lots of different Waves, everything Brainworx/Plugin Alliance, and AudioEase Altiverb. Waves got its shit together (at least technically) like 10 years ago by now, but both Brainworx and Altiverb sorted out their issues pretty recently, with Altiverb 7 being their first release that has run flawlessly on my systems. Or more or less flawlessly; if I resurrect an old project with Altiverb 5 or 6 I better make sure my current version is not in its intended folder when I try to remove the old instances or it will kill my session like a rat-back in a rat-trap… But in itself, it’s rock solid.