You might try using the Based On rule in the paragraph style for the TOC.
That is, create a new TOC style and choose Based On - and select your style with the nested styles.
Then you can reformat your TOC paragraph style to the size you want it and it should retain the nested styles in the original.
I wrote a brief summary to it here https://indesignsecrets.com/creating-smarter-text-styles-long-documents.php
If formatting is applied manually in your heading, i.e. by applying local formatting or assigning a style to selected text, rather than defining a nested style or GREP style to automatically assign the styling, it will be retained in your TOC even if you define a new style for the listings.
Eugene and Peter,
Thanks very much for your replies. I will give that a shot in the next day or so and let you know how it goes.
I really appreciate your time and thought.