I'm confused because you initially said no ceiling speakers since it is a 2 story home? Wires can be run anywhere if you are willing to do the drywall patching and painting, so in-ceiling is an option, but think of the noise transfer to the room above and whether back boxes might be needed. I have Paradigm speakers in-ceiling for ATMOS and they are good speakers. They did not require back boxes; just insulation behind and around them. I found the previous year's model on sale and went with the Pro series as opposed to the Home series.
If you are adding just 2 speakers for surround channels and no ATMOS, then in-ceiling is the last place you want unless it ends up being your only option. You want the surround speakers as close to ear level as you can manage, but given your room that may not be possible for the rear right. Judging from the photos, your main listening position is the brown couch close to the doorway. That means the rear left can go next to the plantation shutters, but the rear right on the other side of the shutters would be too close. That is why William drew the rear right on the other side of the doorway, so that the spacing from the main seat is similar.
The Paradigm di-poles might be an option because they have speakers firing at an angle (one inner speaker and one outer speaker), so the inner speakers will be aimed towards the couch. The one issue is that the rear left will have the outer speaker firing at the wall, while the rear right will have the outer speaker firing into open space. That creates an imbalance but you can either just live with it or maybe add some treatment to the wall next to the big window. Nice thing with the Paradigm di-pole is that it is not a rectangular box. The curved or angled corners give them a softer appearance for on-wall applications and the angled speakers direct sound towards the listener. To achieve the same thing with box speakers you would need brackets and angle the speakers inward, or go with height speakers like the Arendal and SVS speakers mentioned earlier. If colour is a big concern, I have only seen the di-poles in black while the ceiling and height speakers can be found in white.
Below are some of the things that I have experiment with. I started with surrounds in-ceiling between the track-lighting and wall (since then they have been removed and patched). Not the best location, but I did enjoy the added effects and it was worth the effort in my opinion. I moved the surrounds to small bookshelf speakers at each end of the couch, which greatly improved the immersiveness in movies. I experimented with flat panel speakers for rear channels but they were too close and lacked sound quality and dynamics, so I removed them. The current in-ceiling speakers are Paradigm CI P65-R. The new in-ceiling speakers have very low profile grills which is nice. The current surround speakers are not very good quality and are being replaced this week with a pair of True Mini Monitors from Philharmonic Audio and a proper corner shelf.