There is a thread, likely in the Watercooler, where this was asked and some of our monastic members responded. It would be nice if somebody had time to search for it.
I use Bhante and Ayya for male and female monastics I know or with who I talk a lot, and Venerable for others.
It’s worth bearing in mind that Bhante Sujato is Australian and all Australians are casual and informal, so just because he says, “Call me Mate”, please don’t use it on nonAustralian monastics. … Asian cultures are traditionally much more formal, and so I ramp up my formality when addressing monastics from Asia.
Ajahn is used by Thai monks and other monks trained in the Thai, especially Thai Forest, tradition. There’s also this thread that explains that Thai ‘Ajahn’ translates as ‘teacher’ and is used both by monastics who have enough seniority to teach, and also college and university teachers. (When I was a very new Buddhist it was part of my job to go and give talks in Universities in Thailand, and I felt very uncomfortable being addressed as ‘Ajahn Gillian’ because I thought it was only a monastic title!!!)
I wonder if @Snowbird could explain the common useages in Sri Lanka.