Is there any sort of way to get the best of both worlds? to have the PC be able to go from power button to jellyfin server started and still have some measure of security?
Windows with auto login? Not really. That is, anyone with a mouse + keyboard locally can get in there.
You can set up jellyfin to run as a windows service and then it should auto start and run as a particular user without you having to log in. Have a look in the "advanced" section in the jellyfin docs.
Interfaces can be needlessly complex regardless of being flat or skeuomorphic.
But flat interfaces still require mental effort to parse. Especially when the interface is complex and/or crowded and you're trying to pick out active UI elements amongst decorations like group boxes/panels.
Essentially, flat interfaces are currently popular because of touchscreen devices. Touchscreen devices have limited space and thus need simplistic UI elements that can be prodded by a fat finger on a small screen.
But I don't need a flat touchscreen-friendly interface on my non-touch dual 24" monitors with acres of screen real estate. I need an interface that nicely separates usable UI elements from the rest of the application window. That means 3D hints on a 2D screen, which allows my monkey-brain with five million years of evolved 3D vision the opportunity to run my "click the button" mental command as a background process.