1. Should the Boot/OS drive be an NVME?
Personally, I have only one NVMe drive for OSX and for my Capture One catalog files. This way the system by itself "flies".
2. Sounds like the proxy/cache drive should also be NVME. Or should Proxy be one and cache another?
For proxy/chache files I use a single 2,5" SSD, but if I had a second NVMe, I would definitively use this drive for sure for all the cache media. I don't work on 4k timelines like you, but on 2,7k. For the sake of system performance I use cache media all the time (1080p, because my grading Monitor is 1080p as well) within Resolve but haven't done any multicam like you until today. So I don't have any experience with Resolve and multicam. I don't use proxy files neither, because I prefer user defined cache files.
3. Should I use the SATA SSDs in a RAID 0 as an export drive, or is just one drive enough?
It depends on you. Like you said earlier, you work on two projects simultaniously max. So a single 512 GB drive would offer enough space for your export files.
The Hackintosh works pretty nicely on the 2.7k footage I use. It was a little work getting all this stuff toghether in the first place, but in the end it was worth the efford. I just like OSX better and having a 6-core CPU for a reasonable price tag is just sweet. Resolve works so much better than on my old 27" iMac.