I have fairly low ceilings (7.5ft) and a narrow room (15ft), so I struggle to get that large and enveloping sound stage I've heard in some other reference systems.
I've noticed if I change tracks from Stereo to Dolby Surround or NeuralX upmixers, I get some of that envelopment back. I was hoping to sync the upward firing speakers with the mains to achieve some of that effect
Ahh ok, so you're looking more for immersion, not soundstage (soundstage is a quality property of a speaker, not the product of lots of speakers, to an extent). So you want a big sound bubble where you're immersed in audio and there's no longer strict directionality, perhaps it sounds like?
If that's the case, using Dolby Surround Upmixer and spreading out to a 7 channel system with some 2 or 4 channels on the ceiling, would create a big sound bubble.
Upward firing drivers rarely work well without a perfect room for them to generate the effect. You'd be better off with a atmos capable system perhaps and then use surround upmixer. Just spreading the audio across all channels with splitters, DSP, etc, will of course power more speakers, but that doesn't mean they will all play nice together.
Very best,