We live in perpetual summer

I had Callum in one layer, long sleeves occasionally but his arms were long (despite his low weight) so it was mostly short sleeves or no sleeves at all. We never used a hat, but it's over 30 here most days.
At nights he was still in one layer plus swaddled. I used those cotton wraps that are a bit stretchy, not too hot but easier to wrap than a muslin I found.
Stoked that you found all those clothes of Gia's too!