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.