Maybe you can try transcribing a few measures or even a whole piece. This is what many of the great composers did to develop their ears and learn harmony techniques.
That does sound like an excellent addition to the game.

@Benjamin Jack Based on his study of historical composition pedagogy, Jacob Gran recommends audiation of species counterpoint exercises as the royal road to hearing four-part harmony from a score and/or composing without an instrument.

Impressive! What is the highest level on games such as calibrator?

i occasionally have compound intervals mixed with ones within an octave. maybe your games have a biased random generator?

calibrator is imo pretty useless in real world applications, and wouldn't benefit from further improvements, because it's inherently flawed. i've touched on this topic before, but intervals have to make sense in context, within a tonality. C-A and D-B are both major 6 intervals in C major but they feel vastly different to each other. there are other good apps like politonus that put intervals in context and are a bit more useful. transcribing real music is another option.

@Seighart Bui I should have been more specific... it should have related intervals but not within the same octave, such as for the 3 options, P11, P5, P12

@Dima C-A and D-B are both major 6 intervals in C major but they feel vastly different to each other. That's such a beautiful suggestion, I'm writing that down to practice hearing it tonight and see what feeling they both convey.