I don't claim to be an expert either, but actually it'd be fairly easy to devise a workable romanised script for modern Farsi. E.g. there are only 6 vowel phonemes [I think you meant '21 consonants and 5 vowels' rather than '21 vowels and 5 consonants' !] In fact Farsi goes into latin script a lot easier than many other languages. It should be possible to have one without any diacritics, though there would be a number of digraphs with 'h' such as ch, gh, kh, sh etc. The main problem would be arranging the script to fit Dari in Afghanistan and Tajik in Tajikistan as well, since these are basically the same as Iranian Farsi, but with the vowels arranged rather differently.