I suggest to look into Alpine Linux, if you want something which the user can unplug at will (RAM based).
If sudden power drops by the user unplugging are not of concern to you, look into customizing Raspbian Lite, it will be much easier.
Otherwise study what LibreELEC is doing - they are creating an embedded application, just the way you want to do it.
If you are still unsure, you can hire someone to do the work, who knows what to do.
picockpit.com - tools to make your life with the Pi a little bit easier