How to fix navigation interference with image viewing in Windows 10 Photos?

Spacebar hides the toolbar and its state is remembered between newly opened viewer windows. Not sure about if it will reset after computer restart, though.

If you go into Fullscreen slideshow mode, the overlap will fade out after a few seconds when nothing happens. The arrow keys will not bring them back, but the window is full screen.

If you want the same effect in a smaller window, there is a bug present in the viewer you can abuse.

Click on the arrows at the bottom right corner. When it goes full-screen, wait for the image to properly show, then press escape to exit full-screen mode. The controls remain hidden until you click inside the window.

Alternatively, you could download an actual image slideshow program.