Sorry for the late answer, I had no spare time playing with it.

Now I've tried it again and set /sys/block/mmcblkX/queue/nr_requests to 8192 and it solved my problem.
I didn't have to modify anything else.

Now I could debootstrap Debian to my loop device and also I can install applications to the system without any reboots on the device.

It works well.

So, thank you very much your help, you helped me a lot.