Martin, I would definitely try with a powered usb.
I've also a rpi4, but not so many devices connected, yet. For the moment I have connected to the rpi: mount eqr6 pro, guide cam (qhy5L-II) and an electronic focuser. Everything works perfect, but I'm pretty sure I will face some power problems in the moment I connect my future QHY imaging camera. I've read it multiple times. So I will try to be prepared to face some issues. I guess the rpi power supply also could be related. How do you feed the rpi? batteries?
Afaik, the 12v power supply for the qhy camera, it's only for the tec cooler. Nothing related with usb