You said in the same sentence that the source is open and not freely available.
Selling Free Software <https://www.gnu.org/philosophy/selling.en.html>
> Many people believe that the spirit of the GNU Project is that you should not charge money for distributing copies of software, or that you should charge as little as possible—just enough to cover the cost. This is a misunderstanding.
> Actually, we encourage people who redistribute free software to charge as much as they wish or can. If a license does not permit users to make copies and sell them, it is a nonfree license. If this seems surprising to you, please read on.
You can buy only the firmware package as well, without the device. I'll send you the instruction on how to configure everything and which hardware modifications are required.
Edit: Apparently I misread the title.
Additional software (web interface, scripts, utilities) is free, the source code is included in the source package.
The customer is provided with the source code of the firmware components, and with the build system based on mkosi[1] to build the complete firmware: starting from bootstrapping Debian 12 builder image which cross-compiles the packages with additional patches and builds additional software, to ARM Debian 12 image with everything compiled in the previous stage, to the final squashfs OS file and MicroSD image.
It's not that impossible than you may think: several Chinese companies have their own domestic laser printers, claiming of in-house components and development (Cumtenn, ZoneWin), and one company does inkjet printers in addition to lasers (Deli Printer).
You are only required to provide source to the users of your software.
With regards to the software, it is open source but OP is only providing the code to customers who receive the end product. In part, OP is acting as a distributor of the software and is charging a fee for that distribution.
If anyone else gets their hands on that software, they can choose to become a distributor and make it publicly available. It’s their freedom to do so.
A overly simple way to look at is is that OP is choosing (as a small part of their business) to charge for the distribution of the source code but not the source itself.
In reality, it’s unlikely that OP will have a customer who only wants the source code and is willing to pay a fee for the distribution of it. Their customers are coming to them for the service and support.