Sometimes the chord hints (the chords you have available to drag into the measure) can be a bit of a giveaway. To make it most realistic, like you’d have to figure things out in the wild, have an option to remove the hints and instead have a chord search pane like Hookpad has. So if you need a V7, you’d have to type “V7” into the search bar, the chord would appear as a suggestion, and you’d drag it into the measure.
For example here, I think it becomes too obvious what the one non-diatonic chord in the progression is, because you’re shown only two options. So I feel I’m not really learning to hear those chords, but rather knowing that I need to listen for the bass to identify which it is of those two. — Maybe the free entry problems could get thrown into the mix and they would just obtain harder ELO ratings over time. It could even be the same songs. And people can have the option of turning off those problems if they want.
I think this is my top request because of how it causes what you actually learn to be misaligned with what you really want to learn. For example, I’m able to solve problems that are rated 3500+ (in Custom mode) purely by deduction on the chord hints even though I have no idea what those chords are. (You use the colors to figure out the bass, and there’s usually only one chord with those colors in the hints list.)
In an earlier comment I suggested a Hookpad-style search bar. Perhaps a compromise is to always show “buttons” for I, ii, iii, IV, V, vi (on major songs) to allow dragging in the majority case, but then have the search bar below that for other chords. That would be a pretty similar interface to Hookpad.
One more thought: I guess a low-fi approach to this using the current interface is to close your eyes so as not to see any of the chord hints, see if you can figure out the progression in your head, then mark it wrong if you couldn’t get it without the hints.