Nice job. Very strongly reminds me of Aseprite [1] (an offline pixel editor) which also takes a similar approach of layers of frames that can be animated.

The onion skin approach to toggling the number of +/- frames was a nice touch as well.

Recommendations:

- Quickly play/pause the animation with the spacebar

- A separately detachable thumbnail preview to get a high-level view of your current work.

- Overriding the delay on specific frames

[1] - https://www.aseprite.org