For me transmitting any file over the network between my android(Termux) and my laptop(Fedora) is awful. It starts with a quite fast transmission (Like 60 Mbps), but always drops to just a few kbps after a few hundred megabytes. Netcat does this, python HTTP-Server, FTP, scp, everything fails.

... including, I presume, KDE Connect?

I feel your pain, I tried tons of other solutions before discovering KDE Connect. It works well for my laptop/phone combination, but I given how horrible each other solution was before it I'm not surprised if it doesn't work for others.

edit: I wonder if perhaps some part of your phone hardware overheats after N megabytes because it's not built for constant file transfer, or constant file transfer using these protocols at least? What if you try throttling it on purpose?

Yeah, KDE Connect did the same.

I think it is the fault of the network stack of one those two devices, otherwise I can't explain that. Maybe that the devices receive/send data faster than they are able to process. (Is this even possible?)

But I could be awfully wrong

Regarding your edit: That's a good idea, but I never attempted that. I can't test that, because I'm in a Wifi where no device is allowed to talk to another :(

Smells like a TCP in TCP problem.