I’m pretty sure that inflict wounds is your best bet. The 5e spell list is small enough that it’s possible to scan the whole thing, and there’s nothing that deals more damage under the specific constraints you listed (pre-cast true strike, attack roll, not using any daily powers other than the ninth-level spell slot).
I’ve got some space left in this answer, so I’d like to address a related question. This is a question you haven’t asked, but it’s a question which some of the people reading this answer might be curious about: “I’m fighting something that has Legendary Resistance and will choose to succeed at any saving throws I offer. I have a buff spell pre-cast. What’s the most damage I can deal, ideally without walking into melee range?”
It turns out that most of the good answers don’t take advantage of the true strike. For instance, the old standby meteor swarm deals half of 40d6 on even a successful save, so that’s 70 damage plus whatever bonuses you can get from modifiers.
Perhaps our character doesn’t start out at level twenty; perhaps, rather than meteor swarm, we’d like a spell that works great using a ninth-level spell slot but also scales down gracefully. Rules As Written, it’s worth looking at magic missile: there’s an official, though unpopular, ruling that magic missile multiplies your bonus spell damage per missile. Combining this with Empowered Evocation (a wiz10 ability that adds your INT mod to your damage roll) and Hexblade’s Curse (a hexblade warlock ability that adds your proficiency mod to your damage roll), can lead to very large numbers which are technically legal — although, in practice, your DM is unlikely to permit this.
The true strike spell grants advantage on the first attack per round, but a twentieth-level caster might be willing to use a stronger buff, such as greater invisibility. Used against a creature that can’t see through it, this spell grants advantage on all attacks in a given round. With this buff, scorching ray is better than inflict wounds: it fires ten rays maximum and deals 2d6 damage per ray, for an average of 70 damage, or 58.8 damage after applying the 84% hit chance.