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.

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.

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.