The number and placement of the returns are for function, not appearance. It sounds like you have a well-designed system, one intended to operate properly. I wouldn't mess with them, you'll easily handicap the intended air flow. Not infrequently, people buy new homes and ADD returns when doing HVAC work to improve airflow and function.

PS - The ones in the bedrooms ensure good air flow with closed doors. A very useful feature.

Code dictates such things for healthy air reasons. We all have them. No one focuses on them. I did see an idea where a designer took a thin piece of wall board, painted/wall papered to match the wall then put spacers in the four corners then attached it over the duct grates. If you really are that anal about them, try that.

I don't think building codes address the number or placement of return air ducts.

Those decisions are made in the design of each system. Knowledge and experience are involved along with industry tools (to calculate duct sizes, as an example) and with attention to equipment requirements to determine what is needed to handle required flow. But there's not anything more that I know of.

It is possible to remove returns, but then you would need to increase the size of those that remain. Having a return is a bedroom provides good air flow with the door closed. That is a nice feature which most houses do not have.

You could move the vent from the center to the ceiling to a spot above the doorway. It would be less noticeable. Painting the vent cover the same color as the ceiling also makes it less noticeable.

A 25 x 35 family room was built on my house several years before we bought it. It has its own furnace. There are only 2 air returns (side by side in the floor) in the room.

Our HVAC man fusses about those returns every time he’s here checking the furnace. He says we need 2 more.

Could you take a picture? I can’t imagine what a ”terrible air return” looks like so that it annoys you so much.

In addition to painting the vent cover to match the wall, painting the interior of the duct (just the part right behind the grill) makes it even less noticable. While you should definitely seek out the advice of a professional, I wouldn't remove any of them. Our first house was built in 1910 and thus the forced air system was added later. It had inadequate returns and it was impossible to get the temperature to be the same throughout the house. It was very annoying. Since your house is only 11 years old it was probably built state of the art at that time which is a good thing.