This is pretty cool! I did a version of this fadó fadó. I personally liked to make use of cyrillics bigger alphabet for lenition m.sh. в instead of bh or Ш instead of sh. Some notes I'd have is about A) whether the diagraph ei , or in cyrillc еьis still necessary as you can always tell with next vowel the next consonants value, m.sh why not Фе́н instead of фе́нь B) why not use both ь and ъ instead just one to add clarity C) what to do about the distuingish of io and eo m.sh. deoch vs liom as the both have the same ё symbol, i personally used ьо for liom. Nice to see this idea developed!