If the speakers are placed really far apart, then as you suggest you might be better off with mono, but really the best way to decide that is to try it.
I’d your not using a sub then you might want to boost the low end frequencies a bit with a PEQ filter.
But probably the easier option is to use an auto EQ app like HouseCurve
Take multiple readings across the centre of the deck and average them before creating a filter / importing it into Roon.
See the HouseCurve docs, they’re pretty good / easy to follow, there also a thread here on using it.
I’d probably keep any auto EQ measurement below 1KHz as you’re not EQ’ing for a single sweet spot, instead focus on the lower end frequencies which will have longer wavelengths - but don’t be too aggressive, even the low frequencies will have peaks & troughs across your patio.