For reference: half of my school credits were in French and that's about the level of reading I've been at in French for a decade, most of "les romans français" ive read are <300 pages, and not anything mindblowingly difficult or antiquated, usually analized in literature-class format (essays, presentations, quizes on vocab/grammar/etc). Im looking to progress from there, hence the question. (My education is a Canadian "French emersion" one from a border-province.)

moreso wondering how something like fantôme de l'Opera compares to Monté Cristo in vocabulary, or if someone has experiences with originals v translation (in non-mastery / ECL C1 level) second languages, for advice or insight

ie: discussing the nuance lost vs ease of comprehension in translations. Which of the classics would be a good entry into 1800s French vocab, specifically. That sort of thing.

Those aren't necc. any less confusing, for someone who doesn't really get to use French much day-to-day. I find modern slang to be a bigger pain than archaisms in an otherwise straightforward sentence, personally, it's just look up that one word and carry on. And if classics are what someone is interested in reading, they're going to be better off just doing that, than something more modern that will have vocabulary and phrasing the older texts won't use while lacking some of those they do.

Harry Potter in French is very straightforward for learners with a lot of useful vocabulary, but think OP is past needing to read that if something else appeals more.