Why? Keep in mind the T2 flash drive array is much faster than a SATA or even the PCIe RAID set even with SATA SSD’s!
The ideal setup is to get as large of base storage (T2 flash) you can hopefully 1 TB. Setup your MacOS and apps on it (typical setup) leave the rest of the drive empty! Now setup your SATA RAID either internally or externally as your data volume keeping your projects on it.
Also remember this system is designed for heavy pro’s in mind. Video & Music creation being the top two. Others which will likely use this system is animation work and very deep graphics designers. While photographers will also likely use it their need is a bit different as its the XDR display which is paramount! As the color correction is important when working on a series of images that need to match up. So the low end model will likely meet their needs and lots of RAM.