This thread is about air conditioning I thought

It's winter time right?

If it’s over 50F outside it gets very warm and stagnant in the back for the 20min it takes to get off.

In a lot of places we fly it’s over 50F on the winter.

Have the flight attendants completely stopped asking the passengers to close the shades? That small thing can make a world of difference, and opening the individual eye vents.

I’m not sure. My last experience it was dark so the shades wouldn’t have mattered. It’s the lack of cool airflow with all those bodies. It’s just stagnant and warms up fast especially with everyone in winter clothes. The PSU vents don’t do much with just the cabin fans. They need APU bleed air pumping or external air to get the air flowing.

I’m also not overweight so I know it’s not just me feeling uncomfortable back there.

As a commuter and a mostly rational human, I agree with Qotsaautopilot. Run the packs until the last 1/4 remain unless it's either under 50F with no sun (or most shades down) or under 40F with the sun coming in. I'm not with Spirit but doesn't your FOM say something about passenger comfort? One of your own is telling you your passengers are uncomfortable... Also it's just the right thing to do.