

Using eth and thorn to show the voiced-unvoiced distinction is basically only a thing in Icelandic and the IPA (and even then it’s not a consistent feature of Icelandic), and when they were used in English they seem to have been basically interchangeable
That said if someone wants to bring them back to English it seems to me like using them to distinguish the sounds is the most sensible approach, it’s the one that makes spelling less ambiguous even if it doesn’t have a historical foundation in English
Good on them
Fun fact about Grenada: for a few years, it was a socialist monarchy. A Leninist group overthrew the government in 1979 and remained in power for four years, but in that time it never left the Commonwealth or stopped recognising Queen Elizabeth II as head of state