How to set Startup System Security to none with Catalina
Yes you can, as this was a limitation imposed by Catalina.
Is it a known issue that 5.8.4 won't install because it thinks Startup System Security is still on (haven't been able to check yet), where earlier versions didn't flag this at all? I'm on a 2018 Mac mini.
The known issues is Secure Boot must be disabled. What 5.8.4 did was force users to do this, as otherwise they are working with a limited system that has poor write performance and cannot rebuild.
We are workign with Apple to remove this requirement
The M2 has a peak of about 700MB/s per blade, but otherwise will go as fast as it can, up to Thunderbolt limitations. That looks a little slow for writes, but you can try deleting the volume, initializing the blades again and creating a new volume. You may get faster performance after that. SSD's behave differently than HDD's