Solved the problem. Added this to my existing AutoHotKey startup script:
Run, schtasks.exe /run /tn "DWMGlassElevate.exe_-1916372974"
which just runs another compiled AutoHotKey script, now as admin, that simply elevates itself further (to TrustedInstaller status) using PowerRun
Run, C:\Scripts\PowerRun_x64.exe C:\Scripts\DWMGlass.exe
which finally kills the dialogue box!
WinWait, dwm.exe,,15
WinClose, dwm.exe
I'm sure there's neater ways to do it, like a single script that check's its own privilege level, but I don't care. This works . Because it's run first, it means I don't have to hover over each window to correct the initial drawing errors.