With that new information and pics (and further clarified preferences), I’d
still suggest catching Arc on sale/refurb and/or chip in towards your gift or get someone else wanting to give you a gift to chip in.
That enormous vertical space will beg for a great soundbar if you want one. And even if you want low volume most of the time, there will be other times that you might have reason to crank it a bit. Room-filling sound is significantly challenged if you go small here.
Idea 2: buy Arc yourselves and be gifted an
Era 300 or two for surrounds. Black Friday and/or refurb can yield significant savings on them.
Idea 3: Sonos isn't the only game in town (just a very good one for anyone wanted a soundbar-based system). Another good option is the kind that includes a wireless sub that then connects to the surrounds. You can put the soundbar where you expect and then bass and surround sound is wireless beamed to a subwoofer module and 2 surround speakers connect to the sub. Sub is usually put on the back wall for this so that there are no wire runs left & right of the TV (Sub generally sounds just fine ANYWHERE in a room). A good suggestion that is high rated by many sources is
Polk MagniFi MAX AX or
Nakamichi Shockwafe Pro Bluetooth 7.1.4 Channel, though only the former and Sonos directly supports Airplay 2. A workaround for the latter is to lean on Airplay to the AppleTV to then play from the Nakamichi speakers. Again, Black Friday is likely to yield better pricing and/or, be gifted a portion of something like that Polk and then add the rest yourselves.
Idea 4: if you would rather build yourself the traditional Receiver + "dumb" speakers setup, your pictures imply that you could possibly run your wires for the rears around the far right wall (and stairs) to get back to the seating area. There's lots of products to hide these including hollow baseboards and/or various kinds of strips (
example) that sit above or in front of existing baseboards and basically blends right in. Maybe you can get through the underside of the stairs? Or if you have a basement, maybe you can send a few lines down, across and then up again behind the seating area? Harder than a soundbar? YES. But if you can find a way, it will likely yield very best sound.
Again, a good starter- since you are thinking soundbar anyway- would be to go with only front left, front center & front right speakers,
immediately giving you better stereo separation than ANY soundbar choice at any price. Then you can always add sub and rears later. Traditional is not necessarily about "loud"(er) but just best. And in a room with that much vertical (open) space, it would still likely be the best option for great sound- loud or quiet. Else you are asking a LOT of any soundbar and even more so of speakers like HPs and similar.
One more note:
little kids grow fast and may want to crank it themselves in a few years. Teenagers tend to like their audio LOUD. Speakers- if you choose good ones- will likely still be your speakers when they are adults. Consider what you want in 2034 and 2044 vs. only 2024.
All that offered though, if the selection quality bar is dropped to "better than TV speakers",
anything will likely be better. TV makers usually cheap out on built-in speakers. So just anything should sound better than the TV.