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>> Really hoping for some helpful suggestions. I really, really, really, don't want to give up and use KDE5.

Just for info, I ran TDE in buster for all the time buster was testing and never had major issues.
Sure, flickering can come from many places/reasons and your system may be different from mine.
I would suggest trying installing a clean buster *without* any DE at all, then install TDE.
At least if the flickering is still happening, we can be sure it is not related to other DE also installed.
There is a bug open in buster related to flickering (bug 2955 - https://bugs.trinitydesktop.org/show_bug.cgi?id=2955),
although that seems to be very specific to a particular dialog.
Cheers
  Michele
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