Continuing the discussion from Explanation why advocated redundancy is not realistic:
Actually you do not have to do this. It is relatively simple to tweak the TrueNAS install so that it uses (say) the first 16GB or 32GB of the boot drive for the boot pool (rather than the whole drive), leaving the remaining 470GB available to create e.g. an apps pool. (This is what I do on my own NAS.)
And you can do this with either a SATA SSD or an NVMe drive.