Well, lets get it out in the open. Your plan is cockeyed in the extreme.
You say this is for music and not movies, yet you have a projector!
Now those speakers are in walls and NOT ceiling speakers. There is a difference. As speakers go they look to be pretty decent in walls, but I have never heard them.
If you are not interested in a proper multi channel speaker set up then you are best served by using two of those speakers and NOT eight. They should be mounted on the short wall with the tweeter at ear height when seated, which is 36" above the floor. They should be at least 14" from the adjacent wall boundary.
All channel stereo mode is a mess, you don't want to use it. If you put 8 speakers in the ceiling and play in that mode all the speakers will interfere with each other and you will have a mess.
It seems you do actually want a surround speaker layout. Here I have to tell you that is not possible with an in ceiling speaker system. Those are a mess and waste of money and effort.
So if you want a discrete system then place the front three properly LCR. Put the surrounds just behind the listening position with the tweeter 60" above the floor and the rear back in the back wall about 8' apart and tweeters 60" above the floor.
Now if the room is open plan, then put the front three in wall and you could use in ceiling speakers, not in wall speakers in the ceiling for surrounds and rear backs.
I would say that many modern domestic spaces are not really suitable for surround sound, and are really best served with front two or three speakers and forget everything else except subs.
It so happens that I'm building a new home at present and I have designed a new in wall system for out very open main level great room. It is 3.1.
the front three and TL sub are all in wall.
Power amps will go on the right, the rest on the left.
Then the 7.2.4 system gets in own dedicated space.
This equipment is largely transferred from my previous home.
Basically to do surround sound requires a closed space with four good vertical walls. I personally think in other situations 2.1 or 3.1 is just fine. Ceiling speakers are for low level background music, surround at a push and Atmos speakers.
I would seriously rethink your plan.