That’s part of the "embedded computing" problem space, and people have been doing stuff like developing Qt application running as a systemd service and directly

That’s part of the "embedded computing" problem space, and people have been doing stuff like developing Qt application running as a systemd service and directly hitting DRM (the kernel graphical API), so no X, said application being mostly just a webview tailored to your use case. In addition to the webview you could imagine adding an HTTP endpoint to your application for monitoring and remote control, that kind of stuff. I say Qt because it’s popular in the embedded space, and indeed makes doing that kind of stuff easy (by example by having a DRM backend).