I think the nuance is that OP is charging for hardware , software, and service (firmware customization and support).

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.