North facing? South? Not a whole lot you can with single-pane other than using light reflecting sheeting. Any HOA restrictions in your neighborhood?

The exposed ceilings might be your bigger problem. What is above the wood slats? Tile roof?

I tried many different things here in Tucson to reduce heat gain but none of them worked. Finally ended up biting the bullet and had new windows installed.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now