I think many of us will sympathise with how frustrating such issues can be, and that this may well result in venting of such frustrations.
Bearing that in mind, a couple of things: Most any of us who have come up against such a "beating my head against a wall" problem will, with the benefit of hindsight learn that after venting ones' frustrations, unfortunately nothing has changed, the issue is still there. Furthermore, in blowing off steam in a forum such as ours, there is always the risk of losing the opportunity to discuss an issue with others who may be able to offer advice and help, but in order to do so will expect the person with the problem to try to approach it methodically, calmly, provide as many details, heed advice and so on. Ranting and blowing off steam will in all likelihood result in a collective shrugging of shoulders on the part of people who might be able to help, and their moving on.
Sorry for the sermon....
Now, you say the issue started when you moved to Logic Pro 10.4.8, right?
Were you able to revert back to a previous version of logic in which you didn't experience these issues? If so, did it help? If not, consider what options might still be open, how to rule out potential sources of this issue and hopefully narrow it down. It is often the case that some little detail - something in your setup - might provide a clue as to what is causing you this grief. A starting point is to provide lots of info about your system. You have a late 2013 27" iMac with 828 GB used on HD. OK - is that a 1 TB drive? Which sort, fusion, HD, SSD? I don't understand what you mean be "I'm not being offered 10.4.8 on the OS upgrades" - presumably you mean Logic 10.4.8? If so, aren't you already on that? If not, what do you mean? Which Mac OS Version are you running? Which Audio/Midi interfaces are connected to your system, other software, plugins etc etc?
I doubt that I make the same use of the studio horns, strings and other instruments then for example, Jay does, but I have actually used them in arrangements running in live playback situations (live band on stage, me, the drummer, with a click track on headphones and a logic arrangement with instruments and audio recordings of backing vocals and the like running on a Mac Book Pro). FWIW, I never had an issue with any logic instrument.
kind regards
Mark