If the count of apps is small, and all of them are written by you, it's not terribly complicated to pass exclusive access to display across processes.
Only becomes too complicated if you want many apps updating their windows at the same time, or support third-party apps.
> it's less work to just use the already-existing backend.
Short term, yes. Short term, a desktop OS and Electron is even less work.
Long term, desktop environments are huge, and you'll be dealing with long tail of issues related to their features. Each time they break something, you might need to pay hundreds of dollars just to ship the equipment to you for repairs and back.
When a desktop environment is physically missing from the system, along with all related GUI software, services, and libraries, it leaves much less things to break. Users can connect a USB/BT keyboard or mouse, no one gonna handle the events. They can do rude multitouch gestures on the touch screen, nothing unexpected gonna happen either.