Chevy & GMC Vans

I've got a 2013 Savana 1500 AWD with only 87,000 km (54,000 mi).
Vortec 5.3L with 4L60e

The transmission slips a lot. Low speed, high speed ... doesn't matter. At low speed, I can feel a lack of engagement between pedal and road. Up and down shift times are sometimes very lazy. When I get on the highway and put my foot into the accelerator, it's really noticeable. The engine revs to 6,000 rpm and there's little power to the wheels. I have to accelerate gingerly and it will eventually upshift.
There are no trouble codes. Fluid level is correct. It's clean, bright red, and doesn't smell burnt.

It started happening all of a sudden on a freezing cold day. I was late, and driving a bit briskly. It worked perfectly the day before.

Transmission control module failed last year, and was replaced.
Fluid and filter were replaced 4,000 km (2,500 mi) ago.

What can cause this?

My best guess is that something in the valve body isn't allowing the clutches to apply full pressure.

Maybe try a TransGo shift kit?

I took it to a shop. We wanted to try new solenoids, but when he dropped the pan, it was full of scorched fluid and clutch material. I decided to replace with a rebuilt transmission.
It now shifts better than ever before. Night & day difference. Firm, confident acceleration and kick-down. Overtakes easily and no mushy shifts.