I like Drain Bolt. I really do. But since it's a repeating bolt spell, that means that when it's used, it's replacing Prime Bolt. This is all well and good, but Prime Bolt can be teched, and Drain Bolt cannot, therefore making Prime Bolt better to use by a fair margin, even when taking into account the healing of Drain Bolt.
My suggestion therefore, is to add another quest after Drain Bolt X that rewards the player with a formula to craft a techable Drain Bolt X. The formula and spell would be attuned, to prevent anyone who hasn't done the quests from getting the spell. The quest could be centered around gaining true mastery of Drain Bolt, and thereby learning to craft your own version of it.
To keep Drain Bolt from totally outranking Prime Bolt, perhaps Drain Bolt should not qualify for Primal Damage, but it would be nice to be able to put Accuracy and the techs like Romp, Gale, Pierce, and Critical Damage on it. Generally, Romp on my Prime Bolt V adds about 150 damage (and at 35%, it goes off quite a lot), which is the major reason that I do not use Drain Bolt as much as I would like. I am also told that things like Romp do not add to health drain, so having those techs shouldn't overpower the healing.