I don't think it's a simple RAM issue like you say. You can't look at Activity Monitor and see the amount of "free memory" and assume that's the exact amount available to Lightroom. macOS has technologies like compressed memory, etc. that are used to optimize for speed and performance. Your system has 8 GB of RAM available, and at least 4-6 can be used by LR at any given time, if you don't have any other applications open.
What camera are you using? These days if it's a 40+ megapixel image it is a LOT of processing necessary.
Thank you for your thoughts.
You are correct that Mac OS compresses RAM just like many other modern OSes. If you use a tool like "Memory Cleaner" from the App Store you can initiate that compression more frequently or when you choose.
My machine has 8GB RAM installed. After the OS boots up that full amount is NOT available for applications.
On my MBP I also execute programs upon bootup such as Adobe CC, Duet, Memory Cleaner, DNSCrypt and a couple of others. These take up RAM for applications as well.
Lightroom runs OK for me until I use the "Slideshow" module. That's when I get the message about Application Memory being exhausted. I got through that for a while by shutting down just about all the other running programs on the computer. My learning so far is to avoid the "Slideshow Module" on this machine. It's something I only do when at workshops as it's a way for us to share our work.
BTW, my camera is a Fuji X-T2 24 MP and I shoot in RAW and sometimes RAW+JPG.