In a tiny bathroom, closed storage would be more important to me than fashion. Open shelves are for staged, decorative thingys, not the real life function where space is at a premium. Open the doors and be honest with yourself-where will all that stuff, now hidden, go? Of course, open shelves would be pretty and make the room more open, but you live here. Maybe painting the cabinet the same color as the wall would suit you and make the cabinet less prominent?

If it were mine, I would remove the mirror and hang a framed mirror to match the cabinet, replace the light fixture, and take down the plastic grass on the cabinet. A new shower curtain that looks handwoven ( TJMaxx, Target, etc) would give a touch of texture.

I agree that you need to keep the cabinet because it provides storage.

However, the current cabinet is just hulking and so I would look for a cabinet that provides plenty of storage but is less bulky and dark. Even painting the cabinet so that it blends into the wall would lighten up things. One solution might be to have doors with glass but which are opaque so you can't see the contents.

I have a small bathroom and my doors have antique mirror inserts behind mullions - my den has a decorative plexiglass design over opaque laminate which is much "lighter" in feel than solid wood doors would have been and there is no way I would want what lies behind the doors to be visible LOL

Here's a picture and if you look carefully the lower cabinets have antique bronze mirror which hides the contents but provides a feeling of light to my small bathroom. The mirror on the medicine cabinet is "normal" mirror.

This isn't my kitchen but an example of how opaque glass can be used to hide interior but still have a lighter feel.