Also, the user experience is also bound to be much better when a manufacturer provides a tested and supported operating system, especially for “non-experts” for whom a terminal is an arcane inscription tablet.

Mattermost does have an Github Repository with a choice of three licenses: MIT (if using versions compiled by them), AGPLv3 (if compiled by you) or an Enterprise license. I would count that as open source.

A more general business management application like Odoo could work?