Different climates feel different and in some places 90 degrees can feel cooler than other places. In Ponce, PR I could use long sleeves even in days when the day was 95 to 100f but in Florida where I live now I cannot wear long sleeves in 87f. The humidity changes the feeling of the heat. And so you should be more open minded to when someone tells you how fabrics can feel to them. It’s not just the fabric. The place, the body type and every other factor that can vary will affect way the temperatures feel.

Linen.

I’d recommend linen or linen/cotton blend for your shirt, and a linen backing on the waistcoat with cotton (like a brocade or damask) for the front. Although, a summer or tropical weight wool would be nice for the waistcoat front too. You want to at least keep the waistcoat on to be proper around mixed company. For a coat, go for an unlined linen jacket or tropical weight wool with linen lining.
When temperatures get that high, *everyone* is melting. And I think we as modern people feel it acutely because we know and love air conditioning when our Victorian ancestors did not. It’s a struggle to be sure!

I just did a Harry Potter cosplay mashup for Universal Studios Wizarding World of Harry Potter Orlando. It involved TV110 Victorian corset and a modified version of Simplicity’s 2172 steampunk jacket. Everyone kept saying how hot I must be, but I really wasn’t much hotter than I would be in a t-shirt. I made everything out of cotton, so it was super breathable, and the protection of my skin from the sun actually went a long way in helping keep me cool.

That is such a wonderful little fact! Thank you

Dear Lois,

I do Civil War reenactment in the summer. Because of the all cotton fabrics I have very little problems, even in 90+ heat. Is it hot? Not any hotter than when I change into my modern clothes to break down camp. Typical layers for summer are:
-stockings
-drawers
-chemise
-corset
-under petti (only 1)
-hoop
-over petti (only 1)
-light weight cotton work dress/day wrapper dress
-veiled gardening hat

I wear a light weight cotton day wrapper or a sheer for visiting. Same under pinnings, but I add lace or cotton gloves and swap the gardening hat for a light weight bonnet.

And when you’re feeling hot, you just swing that hoop back and forth and it DOES create a breeze under there. (As an educational stunt we dressed a guy one time. It was a hot day. He started swinging his hoop back and forth and was saying, “I totally get this hoop thing now. This is not so bad”. Hahaha! Then we told him he had to do a burpee in his corset. It was pretty hilarious. )

But again, using light weight cottons like were worn during the 1860’s is perfectly doable, at least from my own experiences.

Maybe the fabrics you are using are too heavy?