Terminal safe mode startup command for OS ? – NOT OSx !

Running macOS on a mid-2010 Mac Pro w/3.33 GHz 6-core Intel Xeon & 32 GB 1333 MHz DDR3.

Currently running mac OS Mojave v. 10.14.6.

Beachballing.  Articles on websites keep locking up; won't let me scroll or do anything.  I can read & scroll to a certain point and then the rest of the article is simply not there. Constantly reloading w/shift key + circular return arrow to revive articles. 

Time Machine lets me back up, but won't let me restore anything.

Cannot get into Safe Mode:  Restart & pressing & holding the Shift key doesn’t work.

Using Terminal, the command sudo nvram boot-args=”-x” doesn’t work.  Gives the following message:  

nvram: Error setting variable - ‘boot-args’  : (iokit/common) not permitted 

Hard drive is a Samsung V-NAND SSD 869 EVO.  Running EtreCheck Pro shows hard drive running slow, but hard drive performance test shows excellent.

Drive DX software app shows an almost perfectly healthy disc.