I’m running on this same issue (on a UGreen DXP2800). It seems like such a waste for me to install TrueNAS in a decent size M.2 Nvme drive, when you can’t use any of the remaining space (most of my 1TB drive) for anything storage-related, and I’m taking up one of two M.2 drives with the OS, when I could be using that for more much-needed storage or caching/etc (esp. on a small 2-bay DXP2800), etc.
It is really a bad idea to use the internal 32GB eMMC included (mmcblk0) the TrueNAS OS drive? IIUC the OS stores its config data on the storage pool anyway no? So should the eMMC drive fail, I can always reinstall the OS on one of the M.2 drives and carry on like normal? Or would I need something from the eMMC to restore TrueNAS?