I think you're just running up against the fact you have an 8 year old computer with a very underpowered GPU running the latest version of macOS. Stacking it full of RAM can only help so much. Have you tried a newer Mac rig?
If you have run out of ideas, you might try smart rendering. Offloads all color effects processing via rendered previews, so exports go smoother. That's what I do with my similarly powered "legacy" Macs.
I'll look into the "Smart Rendering" though I can't get that far since I
1. Can't work on any projects in my now stalled, dead-in-the-water, Premiere performance.
2. All attempts to export will fail as long as I am receiving Low Level Exception errors.
I'm beginning to believe that the problem lies more strictly with a High Sierra update and the fact that the GPU is not even giving readings under activity monitor... like it's non existent (non compatible). Everything else so far performs "OK" (namely Photoshop and Illustrator). I've had two cold crashes in Bridge, but not major. I have seen a dip in performance in all Adobe apps but workable and non as crucial as the Premier hang-ups.
I'm not in a dire rush to upgrade my system or even the video card as both were working perfectly fine before updating and I don't have the budget for that now. I'd like to see how performance is being affected and why (particularly after updating) for the GPU, and would understand if it would be noticeably more taxed - but I can't even get a reading like it's just not present.
My son has a 3GB EVGA video card that I can try... have to check compatibility, but I feel like there's something else "off" with the system as a whole. I could even go the route of the Sapphire HD 7950 (popular upgrade for these machines), but even then, have a feeling that I upgrade, everything is clicking, and I still run into the same problem in Adobe.
Is there something I've gotten out of wack, settings & preferences wise?