What you're looking for is precisely what dictionaries in the 18th and 19th centuries did, before there was IPA and phonetic theories had developed. Its remnants are still seen in the transcription system of the American Heritage Dictionary.
The now-seldom-used system is described in detail in unabridged editions of Webster's Dictionary, such as this one:
However, it is debatable how practical such a system would be. It must prove a formidable task for anyone to remember what each combination of a letter and diacritic represents and when a letter without a diacritic is pronounced and when it is silent. Even then, many words would still require respelling, particularly words with silent letters like knit and honest.
My advice is to adopt a respelling system, be it IPA, AHD, Merriam-Webster, or an ad hoc system like "pruh-nun-see-AY-shun".