Selling non-free software under a GNU license != open source. The title will be misleading to most people. Nothing wrong with charging for your software, but you shouldn't call it open source.
Edit: Apparently I misread the title.
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).
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.
You are only required to provide source to the users of your software.