Full disclosure, I'm not playing classic - so someone else can chime in with a better answer, but your team is pretty close if you add a priest. This was dominant in the original vanilla and it looks to be dominant now. This is also the least complex of the bunch. Most likely lead from the Priest and have the Mages follow and do what they are already doing. You get group heals and there is no need for any special macros if your healing on the main window. You can use an addon for click healing to make it easier but nothing from isboxer configuration needs to be done, it would all be in game.
If you add a tank, it becomes more complex but hypothetically more stable on boss fights.