The mirror is too small, and adding "frames" will make it even smaller. Get a larger mirror, don't match the sideboard. ! No frames. The fiddle tree, fine. A longer sideboard ? Even better.

Minimalist means no CLUTTER. Less is more, but you need larger scale.

BEFORE you hammer in a nail. make sure the sideboard is positioned so whatever winds up on the wall, is hit with the ceiling lights (and put a dimmer on the switch).

Idea:

Modern Side Boards / Living room · More Info

Add taller pieces to fill in the spaces around the mirror, because as Jan points out, the mirror is a bit small for the sideboard.

Center the cabinet under the lights a large mirror no frames and I hope that tree is real.Or as Judy has shown

If it is not possible to get a longer sideboard, what else can I do to fill up the space here?

Abandon the idea. Put the sideboard under the window. What is going in the rest of the space? You're hung up on a wall, with everything on that wall, far too small. Move the sideboard. Paint yourself some really large canvas art !!We don't even know what you are using the floor space FOR: ) Get rid of all the dingle dangles except the track lighting. You don't need track lighting shining at a mirror. ANY mirror.