Which wall do you have space for the sideboard? cannot tell from your 2 pictures.

next to the small window(shown in the second pic)

Here's one of many options. Your chandelier seems to be too high, it should hand 30" to 36" above the table surface. Same window treatment for the larger window to which you can also install side curtains.

I agree with a lot of the advice already shared especially about the chandelier being too high and the need for curtain panels instead of blinds. And yes, your paint color does seem to be clashing a bit with the rug. A slightly cooler or more muted tone might help tie everything together.

As for the sideboard, I actually think you could go bigger than 71" to really anchor that wall and make a statement especially since your room is 12'x15'. You have the space for it as long as the depth stays manageable. Here's a beautiful option that I think would work really well:

https://ariidhome.com/shop/sylvie-sideboard/

something like this maybe, If I see correctly the legs on the chairs are black, however I think a fully black sideboard might be a bit too much.

Just make sure you have the room to have a sideboard so as to not interfere with the chairs.

It doesn't "interfere" : )

On a 15 ft wall I'd try to select a sideboard as close to 8 ft as possible.

arched sideboard black

sideboards

A 71" sideboard should work well in a 12'x15' room with a 78" table—just ensure there’s enough clearance (ideally 24–36") around it for easy movement. It can actually help balance the space and complement your table nicely, especially if the style and finish coordinate with your dining setup.