I tried and researched my bluetooth problems for a few hours more: The HZ-RM1 will not be found by ESP32 (BLE library).
I know now this device is "classic bluetooth" but I hoped / guessed that modern bluetooth implementations would also support that...
I had however some success with the camera:
I ordered an ArduCam (2MP model) and got that connected to the ESP32. I tried for some time to use the hardware spi (HSPI) but then the ESP does not boot anymore if the MISO pin is connected to the cam. It works however with the VSPI pins.
I patched together some example code from the examples of ArduCAM and the ArduCAM ESP32 code (old one) and some wifi and webserver examples.
With this the camera is accessible and can capture and provide an image. I serve that with a simple webserver on my local wifi for any request.
The problem(s) I have now for several hours though: How can I guarantee that data transfer is finished before closing the request?
The most I can achieve at the moment - with some luck - is that the browser gets and shows the complete image in the network monitoring view. The browser page itself *always* shows a "connection broken" though.