As a 50-year veteran of visiting Greece regularly there are many islands I'd never set foot on again these days.
Mykonos and Santorini top the list. Horrendously busy in the summer and expensive.
Yet often just an hour's ferry journey away are much quieter islands which still retain their original charm.
The OP's Rhodes,Symi and Nisyros combo is a perfect balance.
The small chain of North Cyclades islands around Mykonos including Andros, are wonderful too.
The best tip is to look at TripAvisor's Greece forums - the islands with the most posts are the least worth visiting.
And this American guy, Matt Barrett, has for many years run the very best website for independent travel there.
www.greecetravel.com