I took T for a while, then stopped for personal reasons that I don't wish to get into right now. Unfortunately, this seems to have pushed my previously high-pitched, feminine voice into a slightly lower range that is broken when trying to speak at any range/pitch. Basically stuck at the beginning of male puberty, I guess.

While taking more T will probably even it out (while lowering it further), as I've said, I will most likely not want to take any more of it. Ideally, I would either go back to the voice I was before, or straighten this current one out (middle to low-ish female). Considering my circumstances and that I was on T for only a brief time (several months with breaks), what are my options? It does not seem that I can train, exercise, or otherwise adjust to this current voice; it's literally broken (in between higher and lower pitches within normal spoken language).

This is something there isn't a simple solution for. Given time, the voice could stabilize or a speech therapist might be able to work with you and find that sweet spot. Other options include having a camera run down there and seeing what state you are in. Possibly a good voice surgeon like Dr Haben giving the pictures could give you some options. Note that Dr Haben preforms more than just voice feminization surgery. As you mentioned. T would finish the job at the cost of a lower voice. At this point, I think you want to discover all your options before deciding on a path.

I would try speech therapy, since it is probably more of a thing that has to do with muscle control and adaptation. As a last resort, if that does not work, you could try a low impact medical solution like LAVA (laser vocal chord tuning) to elevate pitch a bit more, it is not a full voice surgery with all the troubles that go with it. But as I said - I think voice therapy can go a long way there - voice breaks are after all what most of the women here struggle with at some point, also after a voice surgery it is a problem and I believe many have good success with therapy sessions. It does not have to be a trained trans voice therapist for that.