Greetings fellow RPer (or at least someone who thinks like one)! I'm going to give you a bit of an rp perspective. I actually play a thaumaturge myself (poor little Sigurd) who is a voidsent researcher. While in game, he's a BLM, personal storyline, he's just a THM who's way too obsessed with the void and refining his own skills. He actually recently crossed over into SCH-type play, but as he can't quite figure out this ancient Nymian tome he found while raiding a cavern, he's just kind of a SCH without a fairy. But I digress.

I'll give you an interesting fact I've played into Sigurd's personality. He's been having the void whispering in his ear for years. It's slowly driving him crazy as he becomes more wrapped in his research into the void and the events of RP unfold before him. He loses people, he gets stressed out, he's a typical young adult with post-traumatic stress caused by the Calamity. The usual. I'm terrible to him.

Now Sigurd is far from evil. He's Thanalanian (Is that a word? It is now), so he's got his share of shady going on, but he's certainly not evil by any definition, even though he's slowly being consumed by the void. He's just unfortunate. In fact, if you go through the entirety of the BLM storyline, you'll find out exactly what the void is trying to do. It seems more that it would make you sad and possibly self-destructive. He's had moments of severe self-doubt and loathing to a point where he's nearly thrown himself off a bridge, but that passes as he snaps to his senses.

As for your scenario, since both thm and acn can cross into cnj, here's a thought for you and how I decided what classes I'd cross over into Sigurd's skillset.

1. Arcanism is a magic based off math. You can see it very clearly with the scholar spells that this is math-based. (You can see it in Tri-disaster with SMN too). If your character is good with math, it might be a good option, though you need to feasibly come up with a way to explain why he has a summoning book / gem / whathaveyou. If your character cannot count, this is probably not the skill for you.

2. thaumaturgy is based on black magicks, which is basically aether-bending. It is linked to the void but you don't necessarily have to make it absolutely so. Thaumaturges also seem to be incredibly devout to Nald'thal (holds true for BLM too).

Neither is actually good or bad, just with any magicks or DoW classes in the game. It's all about mastery and what you choose to do with the power that makes you who you are in the game.