I'm sorry, I must have been unclear. I am not referring to Excel
strings, I am strictly referring to VBA strings that are multi-megabyte
strings and are therefore extremely difficult to handle with speed.

Have you tried automating Word and using its Find / Replacement.Text method.
Though I expect there are probably better ways.

Don't think most people reading your "large strings" would have imagined
what you were dealing with. Might be worth reposting with subject something
like:
Find & Replace in Multi-Megabyte Strings

Regards,
Peter T