I would def go for a large seamless mirror. I’m not sure on costs but you will want to make sure you use good quality material, this sounds like a “real” home project…not some toy/tinkering project. Don’t cheap out and use acrylic with film and such…you will be using this mirror everyday.
Serviceability wise…that will be tough, unless the monitor or HDMI dies I don’t see you needing to get to the display itself very often if ever. For software you can of course login via ssh/vnc and make all the changes you need to from a remote machine. If there is any way to keep the raspberry pi itself “out” of the mirror ( maybe cut out a small nook with a hinge door and route the cable behind the wall? That will make accessing it a lot easier without having to take down the mirror.
Finally don’t forget to take moisture into consideration if this is going to go in a bathroom!