Hey Ellis, as Darryl said "welcome"...
(Shawnee Hills is a beautiful part of the state, btw...)
I thought I'd let you know that we're also experiencing what you are describing. (have tried all the same trouble-shooting steps you've mentioned as well without resolution)
The only consistent workaround I've found here is to shut down PT, restart the MacPro, then restart PT. That will get me one clean session. Once I switch to the next session, however, random track dropouts occur- tracks are visible, just no playback. All of the sessions I'm working on have multiple frozen tracks.
Some background:
I just started experiencing this bug yesterday after I "upgraded" our system's OS from 10.13.2 to 10.13.6. (PT Ult. 2019.5.0)
We've been on 10.13.2 since it came out, with various versions of PTU- they've all been very stable & the system gets a ton of use. Audio/VI's/fairly big tracking & mixing sessions/etc...
Normally, I do a clean install of the OS & PT to a new drive, but yesterday, for s/w compato & time constraint reasons, I tried to get away with a quickie OS update thinking "it's just a 'dot update', no prob !"
Wrong answer...
Anyhoo... fortunately we've got a system clone, so I'll be reverting to that later today. That's the only solution I can see at this point. Then I'll try a clean install to get back to 10.13.6 when I get some time.
I'll try to remember to post the outcome at that time, but it may be a week or two... just thought I'd let ya' know you're not the only one seeing this.
Lastly, I realize you are on an earlier OS, but was just wondering if you have done an online upgrade of the OS recently (or around the time you began to experience this bug)?