If you’re writing in English, what wrong with Dear Rabbi________,.
Rabbonim and Roshei Yeshiva always went by Rabbi and their English name, i.e., Rabbi Moses Feinstein, Rabbi Jacob Ruderman, and Rabbi Joel Teitlbaum, etc.
Cherrybim suggests the same thing that those informative pages in the back of my dictionary suggest.
In the introduction to one of her wonderful and entertaining books on etiquette, Miss Manners gives herself a chance to answer a question that she has always waited for someone to ask, but no one has ever asked.
How would Charlemagne, the Holy Roman Emperor, address himself when talking to himself. The answer, of course, is “ma majeste'”
When speaking in Yiddish, it is customary to use the more formal German “ehr” rather than the colloquial “du.” In Hebrew it would be “kvodo” as opposed to “atah.” English has no such parallel.