The RDM melee combo is a trivial effort to execute if you get used to just living at max melee range, which is where every job should live anyway because that distance makes it extremely easy to handle outs, donuts, stacks, fans, cones, and basically every mechanic other than "run all the way away" which is rare enough it doesn't need to be the default positioning.
As long as your gear is up to date, doing private solo quest instances on a healer isn't going to take that much longer than it would on a DD, and private solo quest instances don't make up that much of the game anyway. Overworld trash fights will take "longer", but longer in a fight that takes a DD 10-30 seconds is still only 15-45 seconds. As for leveling, healers can level much faster than DDs because instant queues for leveling dungeons.