You haven’t been explicit about what your requirements are but there is a “wish list” topic at Your Purism products wish list and it may help to guide product development if you spell out what would be appropriate for you.
Some general parameters would include:
- Total amount of usable disk storage required
- whether RAID is required
- whether you think an ARM device will be up to the job or you want x86
- how much memory you want
- whether it must be fanless
- whether any video output is required and if so at what spec
- any additional hardware e.g. WiFi (client or AP mode), BT
I have the suspicion that any NAS from Purism is very speculative at this stage, so as far as alternatives go, it is a question of whether you want just hardware (and will install a special-purpose distro or will install a general distro yourself - hence depending on your level of Linux experience) or you want it basically ready to go out-of-the-box - and what your budget is.