I assume "SFF PC" means "Small form-factor personal computer".
The value add is not having to make a large number of technical decisions. IPv4 vs v6, which firewall rules to use, port-forwarding vs DMZ, flavor of Linux, partition scheme, filesystem type, application packaging system, and on and on. For many people they don't care about these decisions, they want "to put something on the Internet" and do it safely. While safety isn't a binary, and engineering is full of tradeoffs, an experienced practitioner can answer many of these questions reflexively and come out with good enough answers for some customers.
In the end the customer should be able to dig in and change whatever they want. But I want to see if flipping the decision dependency around will help. IE, start with stuff that works, then change things, rather than start with parts and make all the decisions before anything works.
My current thinking is the margin on the hardware would be intentionally low, essentially the cost of the hardware %+10 for configuring it a bit, installing NixOS, etc.
The business would survive on support and hosted services. Something like $20/month which gets you access to support to answer questions, help configure applications, troubleshoot issues, etc. Possibly rolling upgrades of your installed software on your behalf. Alerts on urgent security vulnerabilities. Could also handle tricky things like custom DNS (email servers, certificates) and off-site backups. I'm not totally sure what all would be included, but the goal is to make money while providing value, not build a garden or rent-seek.