Esoterica or esoterics?

As plural nouns, I'm struggling to see the distinction. Wiktionary gives:

esoterica (plural only)

Things that are esoteric; things that are impractical or specialised.

And claims that esoterics is a plural of esoteric:

esoteric (plural esoterics)

An esoteric doctrine or treatise; esoteric philosophy.

That would settle the argument, but is also says that esoterics is a different word altogether:

esoterics (uncountable)

The study of esoterica

So what would I use if I wanted to say something like, "I would be good at the English language, but I've yet to master its [blank]"?

The word esoterics does not appear in Lexico (Oxford) online dictionary.

Esoteric is an adjective so it does not become a plural.

Esoterica is a mass noun, and doesn't really describe the oddities of language.

Your choice does not really fit, so I suggest your blank could be

I have yet to master its peculiarities.

Or more simply,

I have yet to master its finer points.