Hello @Jonathan_Drover, welcome to the forum.
I have old OSX 10.13.6, so not entirely relevant reference, but I have always found building RT from source on OSX to be hard. I used homebrew as a base. I have not tried the latest instructions though. My solution was to switch from OSX to Linux (with dual boot to Windows to operate Eizo Color Navigator calibration software). I had many other reasons to switch. My IMac was from 2010 and I wanted Linux.
Obviously OSX builds can be done, since we have these builds: Direct download links to the latest RawTherapee development version.
On the other hand, I find building on Linux (Debian) to be very easy and stable. I have had no issues.
Building on Linux is not helping at all on OSX unless you emulate Linux in a virtual machine. Hopefully an OSX RT expert can reach out.