Can't install Windows 10 update (KB4346783)

The problem I have is that I can't install the 2018-08 Cumulative update (KB4346783). Every time I restart to install the update I get the typica 'please wait while Windows installs updates' screen with the circling dots, my PC restarts, then I get an icon in the system tray telling me that my devise is missing important updates. I check the Update History and I see 'Last failed install attempt on ‎2018-‎09-‎06 - 0x80073712' (see below).

The last time I was able to successfully update was apparently 2018-08-25. Windows defender definition updates apparently work fine.

I've already tried a couple of steps to fix the problem with no success. I've tried using a clean boot (that is, I followed the instructions here). I then tried running the Windows Update Troubleshooter (from Update and Security > Troubleshoot). It reports 'Potential Windows Update Database error detected (fixed)', 'Windows Update components must be repaired (fixed)', 'Check for pending updates (fixed)'. Then I let Windows re-download the update and restart to install it. Same result as before - 'Your device is missing important security and quality fixes.' When I run the Update Troubleshooter again after restarting, it finds and fixes the same issues.

I'm running W10 (64-bit) version 1803 OS build 17134.1, on a personal PC (which I'm the only user of) with an AMD FX-8350 processor and 16 GB of RAM, if that helps any.

Edit: OS build is 17134.1, not 17134.4. To explain, I recently (2-3 weeks ago) had to replace my C: drive and my recovery USB was not the latest version.

Edit2: I've tried the .bat suggested by Run5K, with no success (it ran OK, but updates still don't work.). Although now Windows can't install update KB4343909 ('Failed to install on ‎2018-‎09-‎06 - 0x80073712'). I also tried manually deleting everything in C:\WINDOWS\SoftwareDistribution\Download\, with no effect.

Edit3: Followed the instructions here and manually installed update KB4346783. The log file is here. It didn't work. Windows Update reports that I need to install update KB4346783.

Edit4: So I ran DISM and SFC as suggested by Ramhound. Logs are here and here. DISM got to 84 % then quit with the error 'Error: 0x800f081f The source files could not be found.' SFC reports 'Windows Resource Protection did not find any integrity violations.' Running wmic qfe list lists only two updates installed, KB4343669 and KB4343902. I'm going to pause updates until next month, unless someone has a better idea.