Some simple basics, that have not been mentioned and may not be obvious...
Sound, especially higher frequencies, travels like light... your speakers act like a movie projector
To get the optimal stereo effect (where you can discern the location of items in 3D space... which is the original meaning of "stereo" - ie the illusion of solidity) - the speakers (or at least the midrange drivers and tweeters) - should be at ear level, and circa 60 degrees appart in front of the listener.
This then allows the ear to do its job, with the shape of the ear and its response to sound, as well as the way our brain processes the sound, allowing for our brain to be "tricked" into seeing audible illusions.
Those audible illusions are often termed "imaging" - you cannot achieve a semblance of imaging with most ceiling mounted setups. (there could be exceptions - you might be able to bounce the sound off a wall, if carefully designed, resulting in something approaching ear level sound incoming ...)
Soundstage is another audiophile term, which tends to describe the sense of a large space, not bounded by the speakers, within which the aural illusion then happens ... the speakers tend to no longer be obvious as sound sources... and instead you are enveloped by sound. (if you feel like the speakers are acting as "sound spotlights" then things are not going well!)
Loudness is a whole different ball of wax, it is about quantity primarily and not quality... many speakers are limited in their maximum Sound Pressure Levels (SPL) ie: loudness - and beyond a certain point they will start to distort substantially, or even self destruct....
For some / many speakers, increased loudness can be achieved with a more powerful amplifier - keeping in mind that doubling the loudness (circa 10db) requires roughly 16x the amount of power...
So if you want to double the loudness of a 100W amp, you would need a 1600W amp (and a speaker that can handle that much power!)
In most cases, loudness is best increased by using more efficient speakers - the specifications for the speaker will state something like 90db/wm or 90db/2.83Vm (ie with an input of 1W or 2.83V, the speaker will output a loudness of 90 decibels measured at a distance of 1m)
My speakers are rated 86db/wm, highly efficient speakers can be rated at 96db/wm - and will obviously be twice as loud, for the same signal input from the amplifier.
So if you are stuck with the speaker locations, and seek more loudness primarily, you should focus on more efficient (loud!) speakers.
Alternatively, if you are not concerned about imaging and soundstaging, are happy with an overall ambient sound effect (quite common with ceiling speakers) - then you could add additional sets of ceiling speakers... each additional speaker added should boost the base loudness by circa 3db.
In terms of room layout, cabling etc... we are now in an era where many speakers are connected wirelessly... so if power is available to the speaker location, there may well be alternatives that do not involve speaker cables (but involve power cables instead)
I hope this helps