I was able to work out a way around eventually, the guys at the ESP32 github suffered through it: Task Watchdog & AccelStepper · Issue #2892 · espressif/arduino-esp32 · GitHub
It actually turns out to be an issue with the AccelStepper step function using delayMicroseconds, which doesn't register with TWD. Any delay type that registers will make your motors very slow, so right now there's no easy way to feed it. With TWD disabled on the stepper core, you will still get reboots when you come to the stepper.run() function in a loop - there seems to be some timing issues with the task switching. So the solution for now is to disable TWD on on the stepper core and set that core's task to higher priority than the other.
And that's it for now, works fine. I think they're gonna keep working on it, but it's fine like this for now I suppose.