I still don't know how to make the sdcard appear directly on my desktop computer when I connect my phone, but I have a workaround : symlink the sdcard mount point in the /home/phablet directory ( = "internal storage" on the desktop computer).

This is easily done using the terminal (or adb shell). Find the exact mount point of the sdcard using the command mount; it is something like /media/phablet/XXXX-YYYY. Then enter ln -s /media/phablet/XXXX-YYYY ~/sdcard

Since it was working in android, there must be a way to have it working in UBports. I had more or less the same issue in the Xperia X port and could make it work (see here). Turns out that the proper device to use was usb0 and not rnids0 (but editing some files was still required to have usb0 being available). If you are comfortable with linux command line and modifying bash scripts, you should try things along what I did.