I would consider getting a round or oval

I like the size and shape, since the sink is squared off. if you're going for farmhouse modern, then I like it. if it's too rustic for you, I'd suggest the same size but with a matte black frame.

if you end up painting it black you might not like it because of the frame, it has a heavier look. Find one with a slimmer frame and different metallic finish.

yes, to clarify I meant the same thing as what Claire says. either keep the mirror (it looks fine to me), or get a new one with a matte black frame, in the same shape and size, but with smooth (not rustic) wood.

I think the space could benefit from a larger more striking wall decor.

I think the space needs a larger light fixture. The fixture above the mirror looks small compared to the mirror. I would not paint the mirror.

I also agree with @BeverlyFLADeziner regarding the art.

or instead of art you can place open shelves above the toilet

The mirror is fine. You just need to lower the artwork a few inches, and consider reframing the art in black.