Thank you for being such a polite member of this forum who has really taken the time to read the forum Requesting Help guidelines and who does not waste @moderators time.
EDIT
Your post above is not really the solution to your questions. Therefore I have unmarked it, because someone else in the future might read this topic and think that this is the way to create post thumbnails on list pages.
Front mater parameters used to be the way to define image thumbnails… back in 2017.
However since then, Hugo has a feature called The Image Resource that also comes with Image Processing, these are the methods one should use to create post thumbnails on list pages.
Front matter parameters require manual work. Page Bundles along with naming conventions for filenames do not require manual work.