I use Mutagen personally but It's not capable of doing what I hoped

Mutagen is OP.

Not really, but its an outstanding heal.

I do think DKs don't make very good healers personally, but I'd be happy to be proven wrong by a great build. They just don't seem to have any magicka upkeep or passives that synergize with healing.

I'm not quite up to VR levels only at 45 but I've had only had occasional issues with my DK healer. Your complaint isn't really with DK but rather with resto staff. Our best single target heal is Steadfast ward morphed for the residual heal. People at low health are actually easier to keep up in some ways since the 300% bonus provides a huge boost.

As for difficulty grouping you may want to look at what your tank and group members are doing lots of damage can be avoided and if the 'tank' is just wailing away expecting you to keep them up they're likely to end up dead and you feeling somewhat impotent. As nice as big heal might be to have the reality is it would be more of a oh *** button then you'd be gasping for mana and a dumb tank will still just end up dead.

Some groups I barely had to heal as the tank knows how to block and move to avoid damage and others they drop like a fly as they charge into 15 mobs without waiting for the group and just start swinging away expecting some miracle to keep them up.

As for advice with Dk, if you don't already The dragon standard morphed for the 30% damage reduction makes most boss fights trivial if I have it charged up. If the fight is more mobile use magma armor for the large damage shield and free mana fill up. Make sure siphon spirit is always up on the boss. Our best single target healing outside of ward is Grand Healing, make sure its morphed for the extra one second tick. It should be healing about 100 hp a sec, keep regeneration going for another 50 hp a second. Spike damage is handled by ward. When tank hp is high make sure you're refill your mana by attacking the boss with siphon spirit on it. Use obsidian shield when mana levels permit to preshield incoming damage to the group while health levels are high.

synergy isnt as bad as people make out resto staff healing is effected by weapon damage and igneous weapon buff provides a nice 10% healing boost or so just off the buff to weapon damage and the flame damage effect helps speed up boss encounters its quite noticeable when it drops off my team mates. Obsidian shield is a nice long lasting damage shield that effects the entire group if they're close and ash cloud is nice for mitigating damage on large trash pulls or adds on a boss.