What is a good way to change desktop background on system startup?  

avada said:

So I tried this.
The image change via this app is also afflicted by the issue of lag and flickering GUI's. So it seems the glitch is in Windows' changing of the accent color. For whatever reason.
This might work out though, if it successfully runs and changes the background at startup. (Though it's a bit funny when I set both change on startup and close after change. :) ) Do you think it actually runs at system startup, or more like user login?
(If the latter I might try to start it on actual system startup via Task Scheduler, because of separate issue. Though that almost always fails, if it ever succeeded for me. I can't recall an occasion)

(speaking of: have you ever hear of a sort of third party accent color changer app? Which would generate more pleasing colors?)

I have not seen or experienced the first- I use laptops- perhaps there is some issue with your configuration or your PC being a desktop.

The program runs when you log on as part of the startup sequence. You will see the old background after loggin on, then when the program is run, the new one.

I have the settings thus:

using a folder of suitable images.

As to your last: one of Stardock's products might be relevant: e.g. Curtains



Or consider this:
Add Personalize (classic) context menu in Windows 10

avada said:

I see, that requires some patching (or other hacking) to work, right? So that third party themes can be set to be used.
I have a number of pictures though. Creating a theme for might require exorbitant time.

DeskThemePacks are used in Windows. No external software required.