How do I specify a mount point for an attached disk image

with some experimentation I found out that I can attach/mount a sqashfs disk image (created with gensquashfs on another linux system) with the disk tool via its menu “attach disk image”. It is mounted by default at “/run/media/$USER/disk”.

Is there a possibility to specify a different mount point?

solved it myself: In the Disks tool, I can specify detailed mount options by clicking on the “Additional Partition Options” and then selecting “Edit Mount Options”:
only the option “Mount at system startup” does not function as I would expect it.

I still have to open the file manually in the file manager with the “Disk Image Mounter” despite having checked “Mount at system startup”; does anybody have an idea why and/or how to automate the mounting?