Sounds expensive. If you end up replacing any bricks, it seems like it could be a challenge to get a correct match.

There is a part of this article that talks about black windows with darker brick - you may want to read it.
https://www.mariakillam.com/black-windows/

I would reconsider. This is going to be very expensive and require a lot of brickwork... which equals $$$$. Have you gotten bids?

We had an approx. quote from the guy that installed our garage doors, and because the rounded top windows would have to all be custom, he said doing the 3 rounded tops, were just as expensive as all other windows combined,

So, since I would prefer a more modern look, opening the top to square them off, and paint or trim the brick, may be the look We want at the same price.

Did you recently paint the soffits that grey color? If not, when you do paint consider a dark putty tone like the mortar which will work better with your brick tones. And after reading the article I linked you may want to consider that same putty tone for the new window frames - square or arched.

Is the light color brick still available. You might be able to salvage some of the bricks, but I think You'll need to replace some as well.

If I had to guess, the cost of modifying brickwork will far outpace the cost of the arched window units. You will definitely need new brick of one color or the other if you don't have any extra (more of the light if you make the window taller or more of the main color if you make them shorter. And between possible brick color variations and definite mortar color variations, you will always be able to tell that something was changed. You could infill the exterior with black painted wood (or aluminum). You will still need to make some drywall and trim modifications to the interior.