I’ve been building something in this space as well, but with a broader toolkit [1] approach rather than a single-purpose interval trainer. The thing to me that's a bit jarring is the keyboard itself. Also, when it comes to ear-training (and while I've included this as well), without context it's not quite as useful. It's an area I plan to revise and really work on, but want to find a better approach. Good luck.
[1] https://www.umaro.app