I had a 2017 MacBook Pro and two 27 inch 4k monitors. I connected these through a variety of different dongles and configurations but always had a similar issue - after being in use for a while, no matter what I was doing, the system started to thermal throttle to the point of becoming unusable.
After many hours of hair pulling I discovered that using display scaling was the issue. Apparently using anything beyond the "default for display" option was taxing the GPU, which causes heat, which then heats the rest of the system to the point of it throttling the CPU. When I finally realized this I moved to using an eGPU. So basically something like 700 bucks for a Razer Core eGPU and AMD video card to function as a dock. Oh and cherry on top is the Razer Core's NIC doesn't work in macOS, Razer knows this and isn't planning on fixing it.
I was really excited about the M1 systems but I absolutely do not want to go through the hell of figuring out how to get two displays working. Particularly considering that I can't just sidestep the issue by using my eGPU since those aren't supported.