At level 56, Holy should definitely be incorporated into your DPS when facing multiple enemies. It's also a good defensive tool because of its stun. You also have Assize, and soon Aero III to add to the mix. Healer DPS is simple compared to a real DPS class, so it will never feel like you have a complex rotation, but it should make it a little more interesting.

If you're finding dungeons too easy to heal, you might like some of the harder end game content like Savage, but you need to get through the story first. Regular dungeons usually aren't very demanding and may not make much use of skills like Cure III.

If you're not particularly interested in WHM, I think it would be best for you to try another class. SCH will level SMN at the same time and give you two classes to play of different roles. I think SCH is also a little more engaging than WHM once you get midway through the game (level 40+). I enjoy managing AST cards and you'll start at level 30, which will save you a little time in leveling. Also remember that you get an XP bonus for classes at levels less than your highest leveled class. So this will ease both switching from WHM and returning to WHM later if you decide to give it another chance after you level a different class.