Seeking in HEVC media has flaws and some hangs are inevitable (as you found, stop and restart playback generally works). I can't explain the source changing issue, but I don't see problems with source changes on any of the boards I test with and my own receiver, and it's not something other users are reporting against AMLGX images.
Someone is rewriting HEVC support, but until I see code to test I can't say whether that's a solution, and there's no timeline.
Why don't you just let the community do the "hard" task and let moderate the forum? Like it's kind of standard nowadays so that users just flag/report content and it will be invisible until a mod approves/removes it?
That way the mods should have much less work and users might be happier because their legit posts are actually visible and not in limbo for hours or days because only 1(?) mod actually does this job.
Relying on users to flag spam doesn't work because when you present a shitty task to a large audience everyone hopes or assumes someone else will do it, with the result that nobody does it, except for the staff/mod users (who are already doing it, and the people doing initial post approvals and the main responders to around 2/3 of the threads in the forum anyway). So it's not really any more or less work for staff, and it's a highly effective spam block. TL/DR; It works, and despite the occasional complaint (which like busses, come along in threes) it's not going to change.
ðmac {
status = "okay";
phy-handle = <&internal_ephy>;
phy-mode = "rmii";
};SEI510 uses the internal Ethernet PHY embedded in the SoC itself so as long as that's present in device-tree Ethernet works. The X96-Max dtb uses external PHY which requires different content and the correct compatibles to work. The SEI510 has special audio config though, which will not align to the config used with most TV boxes.
For WiFi, the SDIO bus supports discovery and probing by the kernel so as long as the bus is enabled (and is most device-tree files it is) the chip can be seen and matched to a kernel driver and used. I've added the RTL8189ES/FS drivers into my private LE sources for a while so the needed driver is present in the image. Those vendor drivers are deliberately not present in the main LE releases, and that's not going to change. You can find the packages here: https://github.com/chewitt/LibreELEC.tv/commits/amlogic
It should be fairly simple to create a device-tree for the box. Start with the X96-Max and just change the Ethernet config.
Stick with the q200 dtb if that works for Ethernet. Now put Kodi into debug mode and run "pastekodi" after demonstrating a video that doesn't work right and share the URL generated.
To see whether this is an issue with the settings add-on or somewhere lower in the BT stack, SSH into the device and connect/pair the device using "bluetoothctl" instead the LE settings add-on. If that works; the issue is in the add-on, but the pairing/connect should be persistent now (workaround). If it still fails, the issue is likely lower level and the first step would be updating to (or testing with) an LE12 nightly where newer code/drivers are being used.
You probably need to self-build, so these would be suggested reading:
https://wiki.libreelec.tv/development/build-basics
https://wiki.libreelec.tv/development/build-commands/build-addons
"You can please some of the people some of the time" 
You're probably better off asking the Q in the Kodi forums, since DLNA capabilities are Kodi features and not really specific to LE.
Kodi caches versioned addon installer zip packages in /storage/.kodi/addons/packages so if you (re)install a package which has already been downloaded and is present in the cache it will be installed using the cached package and will not be (re)downloaded. Kodi also stores add-on settings under /storage/.kodi/userdata/addon_data so when you delete something and it asks you about deleting data on disk; that's what it refers to. If you don't delete the data on disk you can delete the add-on (will not be present in /storage/.kodi/addons) but if you later reinstall the add-on the previous add-on settings will be reused. Since DVB client add-ons interact with DVB server/source apps; some config may be persisted upstream too, e.g. channel layouts and EPG settings.
video=HDMI-A-1:1920x1080M@60^ Adding this to kernel boot params (the APPEND line in syslinux.cfg) forces the video output to the HDMI-A-1 adapter. Set this if wanting to use HDMI and not the internal screen (LVDS) or change the adapter details to suit the second HDMI device and it will be used, else the kernel will detect/use the first device probed (on a laptop, normally the LVDS connection). You can also use content like "video=HDMI-A-1:d" to disable the HDMI-A-1 device (allowing another to be discovered) .. whichever works is good. There is no F7 style hotkey to swap outputs once Kodi has started.
Run "dmesg | paste" over SSH when booted from the Q200 and Q201 device-tree files and share the URL generated.
I'm in the UK for a while, so will see if I can find a second-hand U9-H for testing .. no promises.
At the boot menu .. hit a key on the keyboard (connect a USB one if needed) and type the command "live" or "run" then hit enter. If you do nothing it defaults to "install" but this requires you to select and confirm the target USB/HDD device to install LE to. Install repartitions and formats the target device exclusively for LE use (no installing to a partition like some Generic distros do).
The main challenge will be booting the device without an SD card slot and no UART cable to show you what's actually happening in the early stages of boot when you need to trick the device into booting Linux not Android. In practice; that'll be damn complicated and I'll say "no, not possible" simply to avoid the 500 hours of blind guesswork required.
You're over-thinking the problem. Just buy the 65"+ 4K TV that makes sense for your future 4K viewing needs and budget. In 99% of 4K setups Kodi is still running the GUI at 1080p and only switches to 4K when playing 4K media so it's basically the same experience you have today except for the minor size difference between a 58" and 65" panel. The TV will still handle upscaling from 1080p to the true native 4K resolution of the panel, but it's still basically playing the same SD media at the same 1080p. In theory the 1080p pixel blocks render bigger on a 65" vs 58" so it should look worse, but since colour technology advanced by a mile since ye olden days of Plasma screens so you won't notice that; you'll only notice the colours are wildly better.
NFS has a reputation for being "lighter" which is great for low-bandwidth crap WiFi but SMB is often more resilient with dropouts and other forms of crap WiFi. There's no universal answer as WiFi is inherently a bit crap, and RPi4 WiFi is at the low-expectations end of on-board WiFi implementations so it's best to experiment and see what works best (or least-worst) for you.
If everything works briliant when you run an Ethernet cable you can join me (and the rest of staff) finger-pointing the WiFi connection 