Nobody uses it. You’ll be the only one.
What for a pile of crap of an opinion. Most languages can be described as "nobody" uses it. When you are outside of the top 10 languages in the world, then any language is a harder way to use it. You could move to the country or region where it's then spoken or maybe find the community in your city which speaks such a language.
Esperanto can be also found like that. There are people who speak it and also there are regular events where people can meet up.
Esperanto is eurocentric, because it's international. Because romance languages where made by colonialism of the roman empire. The argument goes of "equality". Thinking the other way around would be that asiatic languages colonized the world, then Esperanto would be based on asiatic languages.
Esperanto is a pragmatic language, not a "totally neutral" language. If you design a language to be "totally neutral" then parts would be distributed differently. How to chose which vocabulary of languages should be used often?
So using romance languages is a pragmatic solution to this. Through usage words can be added or fall out of use, all that is allowed in Esperanto and which can make the language out of colonialism in the future more egalitarian.
But it's ignorant to ignore Esperanto at all and morally vilifying it as "eurocentric therefore bad".
Saluton, kiel vi?
They are equally useful and about as well developed.
Comparing Esperanto with Klingon is comparing apples and oranges. No, Klingon is not that developed as Esperanto is. Klingon is for a bunch of enthusiastic Trekkies who wan't to live their universe. Esperanto has a 136 year old history with a bunch of literature, music and tradition behind it.
No other constructed language has that.
Esperanto is also good, but when my partner tried to learn it, they were weirded out by some of it’s quirks, like noun declinations based on whether it’s a subject or object, that seems unecessary.
That sounds interesting. Esperanto has no noun-declinations, it's an agglutinating language, you don't bend words (= declination).
But what is barely resembling that what you mention is the two cases of the language, which is nominative and the so called "accusative". Which is adding -n to words to make them an object, depending on whether the verb of the sentence needs one or not. This case also is not just for objects, but also for directions, for measurements and time. That combination normally confuses the heck out of people.
Which is why there is also an in-joke in the Esperanto community "don't forget the accusative", because people forget it or apply it too often.
Every book is not "accessible", when it's not even opened and willfully ignored of existing.
There is:
There are languages to which it's less accessible, but from the bigger ones, it's quiet accessible.
But if people don't open their eyes they don't see the forest in which they are standing.
I speak Esperanto for 14 years now. And no, "si" is not a singular "they". That's a self-referencing pronoun. And if that usage is used for genderless addressing a person then this is simply incorrect usage, because people don't know how actually the language works. It's used in sentences like "li lavis sin" vs. "Li lavis lin". The first one says "he washes himself" and the second says "he washes him", the first references the person who executes the action to reference and the second says that the action is done on a different person.
If it comes to Esperanto and genderless usage then there ĝi (it) or ri (they). The first one would be more in accordance with the fundament of the language and the second is a new pronoun which is around since at least the 70s.
No need to misuse si.