Ice cubes in the water before mixing – filtered ice. Or those plastic blocks filled with gel.
There's a kind of picnic cool box that connects to your car output. (Mains plug included.) It provides modest heating or cooling by electronic means. It's not a conventional refrigerator. I put my chemicals into it some time before I need them and monitor the temperature from time to time.
I begin processing with the temperature too low and hope that the drift-past theory works. A more diligent person would do proper tests. If we had more reliable heatwaves, I'd do it.
The photographer has to suffer, but he can walk out.
Mine's down the garden in a stone/breeze black shed and well insulated walls, floor ceiling and door, as long as the door's not left open long it's staying cool, I've been leaving a small fan running at floor level and that helps a lot.
I've been working at 26ºC this last week or so, but that's what I was used to in Turkey so it's nothing extraordinary.
My darkroom runs about 81-84 degrees F during our long summer months. For film processing, I use a tempered water bath when developing film in tanks or a "ice brick" in the upper water trough of my Jobo. I don't try to print analog silver prints because the print developer temp causes very short development times. And, you can run into "gelatin issues" at high temperatures. That said, I reserve my pt/pd printing for the summer months because I develop these prints at about 110 degrees F. So, silver gelatin in the winter; pt/pd in the summer.
Btw, love that book you have the thermometer sitting on!!