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.

@gpatel-fr thanks for digging into this. So, right, it's supposed to use ncm and not rndis, but usb-moded should handle tethering with that .
The file you tried to cat is restricted to root access, but even with sudo, it's not accessible, and that probably explains why the configuration script fails, falling back to charging only.
Maybe the fp5 is missing the corresponding kernel module now, while it had it back in August?
I created a ticket.

I finally nailed it! After a simple change and recompiling a package, I could have gps working continuously for over 3 hours, which had never happened previously on my FP5.

The problem was indeed in ubuntu_application_gps_hidl_for_hybris.cpp part of the package libubuntu-platform-hardware-api3 whose repo is at https://gitlab.com/ubports/development/core/platform-api, and lomiri-location-services calls this library to access the android drivers (through hybris). The problem was that the code continued to use a chunk of memory that had been supposedly freed (use-after-free) and the memory corruption only became visible when the allocator reused that freed memory chunk for a new allocation. At that point two pieces of code competed for the same memory, leading to crashing. The issue is more likely to happen quickly when many applications are running, explaining the somewhat random duration before the crash.

On my Xperia X, a different piece of code was used (ubuntu_application_gps_for_hybris.cpp, not ubuntu_application_gps_hidl_for_hybris.cpp) which explains why it never had this problem.

That gps_hidl piece of code has not been touched since 2020 and I believe UBports uses it in all phones running running Android 10+ vendor blobs, meaning that the problem should not be specific to the FP5. Yet, I find it really strange that no one reported the issue previously and that only @WillemHexspoor confirmed it here. Does that mean that UBports users either never use their gps for extended periods (e.g. routing with PureMaps or uNav) or that they only do so under waydroid, or that other phones access the gps through a different scheme (another library)?

I have filed a merge request to that package.

EDIT: I take back all what I said yesterday, it eventually crashed all the same after (except after a very long time (~10h), and then this morning again after only 3h... So the root cause is not yet found... I'm digging further.