Oh, one thing I was curious about is if there can be spaces when you’re mounting a drive. Should the share be entered as “My Book” or would it be “My_Book” ?
So space sign is legit.
A modern web browser is a media player on its own. So it goes to a wrong direction on LE. If one needs Kodi and a web browser, install a regular OS like Ubuntu or Raspberry Pi OS.
Skin developers are responsible for compatibility. Ask at Kodi forum, where those devs are.
My NAS is permanently mounted on Ubuntu. I've used this guide:
https://ubuntu.com/server/docs/how-to-mount-cifs-shares-permanently
When I connect it to the AV receiver however it can't boot further than to the LibreELEC splash-screen. I didn't set LibreELEC up to only output 4k or something. LibreELEC worked fine in the otherwise same setup running from the Raspberry.
So your current setup is NUC -> AVR -> Projector, and former setup RPi -> AVR -> Projector worked.
We had cases where an HDMI firmware update made things work. Make sure it's up-to-date.
I decided to upgrade the hardware from a Raspberry Pi 2 B to an Intel NUC 13.
The NUC provides a higher resolution by default, compared to RPi. There is a chance that your AVR can't handle such a resolution, and video pass-through doesn't work anymore. So test another setup:
NUC (first HDMI port) -> Projector
NUC (second HDMI port) -> AVR
SMB is a file service, which works on top of a block-based file system. So in theory the underlying file system doesn't matter. However, I recently switched from exFAT to Ext4 on my NAS, since I had trouble to access files after a while. I guess Ext4 is easier to scan for errors under Linux, and such scans are triggered periodically. I suggest Ext4 for Linux.
SMB default port numbers are 139 or 445, so that's OK.
With projector and AVR connected, run getedid create from SSH, and reboot. If the issue persists...
Are there any plans in the future to replace X11 with a more up to date window system?
No.
We currently do internal tests on LE 12. No issues so far. Maybe official release next week. 
You can configure LE as NAS, or you can autostart Kodi on Raspberry Pi OS.
With projector (at HDMI-0) and AVR (at HDMI-1) powered on, run getedid delete and getedid create again.
Do you get an error message after one of those commands?
And I think any OS (including the linux too) is good to be rebooted periodically, so then why not every night... Less issues... Less headache.
I'm using an RPi3B as an always-on NAS (only Ethernet, Raspberry Pi OS). No periodical reboot needed. It just works. I suggest to figure out, which add-on breaks stable usage.
I can log in via smb to the LibreELEC-Generic.x86_64-9.2.6.img version works
Not helpful. Continue with the suggestion from chewitt :
If no, does "live" or "run" modes work (running from USB)?
If it does boot with a black screen, you should be able to login by SSH afterwards. In that case...
Well same here, Im using LibreELEC-RPi5.aarch64-11.95.2.img.gz with a RPI5...
Use the latest nightly build instead: Download
Did you tried Generic-legacy variants? That's needed for Nvidia graphics.
