FinderMinder is the answer... with some deeper explanation.
Download here: FinderMinder
In FinderMinder's "Contents" (right click the app, select "Show Package Contents") you will find a "Contents" folder. Open that, then open "Resources" then you will see a file "com.apple.finder.scpt".
Open com.apple.finder.scpt in the native "Script Editor" app. Delete the
(* and the
*), change the
set toolbar visible to true to
set toolbar visible to false, then save the script. Now run FinderMinder and set it up normally to allow it to control the Finder through the System Preferences. Voila!
On and off for years I have I have been searching for a way to remove the Finder's sidebars and toolbars, never to show again. Unfortunately the "bar" epidemic became a pandemic with Catalina which randomly likes to re-show sidebars and toolbars on windows where they were turned off. Finder has been a hot-mess since the inception of Mac OS X and FinderMinder provides some serious relief from the highly frustrating experience of Mac OS X Finder. (FTFF)
Props to @1.61803 for the original FinderMinder tip, you are my freekin' hero! This might be the best Christmas gift for me for 2019!