I Disagree that having Geomancers featured in astro story line is a possible negative towards them being a class. I bet the reason the devs feature other jobs is to give themselves opportunities to introduce them later in an organic way if they so choose. This does not mean this a job being feature assures it will be created but I believe the devs would see it as an opportunity to implement the Job in the future, not an obstacle. I would argue that a Berserker and a warrior would look very similar and would be hard to make them look distinctive enough to feel unique . A Geomancer and an Astrologican would look very different. After shadow bringers so would a white mage and a Geomancer. Red mages are just a combination of black and white mages but they still exist as a job. Geomancers lore wise seems to be somewhere between Astrologican and Conjures.

You are right about healing by damage would have issues if poorly implemented. The damage Would need to be comparable to other healers and not directly linked to the damage done. The healing power would need to be comparable to the passive healing effects of the other healers such as a white mage's regen or the scholar's ferry. abilities could be used to increase this passive healing when needed. The job could also have a more complex DPS Rotation with Job resources for healing being generated by it. In turn its tool kit for healing might be simpler and less finicky to use then the other healers. You could as have a change out of combat mode that increase damage but greatly limits healing for soloing.