(1) In case things get worse - now or when 1903 arrives - make a Windows system image backup. I finally had to restore one, & it went quick & well with hardly a decision to make (on my small & simple system)…
Restore Point with External Drives
(2) I like Douglas's suggestion. Because you & the site are both 1809, it amounts to an over-install of 1809. You get new system files, but 3rd-party apps & user files remain untouched. This is usually the next step after DISM & SFC failed to be a fix.
(3) Well, did you see what it said about the Servicing Stack in here...?...
https://support.microsoft.com/en-us/help/4493509/windows-10-update-kb4493509
April 9, 2019—KB4493509 (OS Build 17763.437)
You can check whether you've got the right one at "START, Settings, Apps, Programs & Features, View installed updates"...