I think you've already received a bunch of good feedback on the frustrations that this game gives, especially in relation to the jump, so I'll add in a few other things to consider when updating:
1. Less about the jump tuning, but however you track if the player can double jump ignores if they just walk off a ledge, which means the player can delay their jump to be in midair. I was able to use this to cheese a few levels.
2. This may be intentional, but guiding the player towards the diamond but not the stone feels like an odd choice. The level becomes a bit of a treasure hunt at the end, and frustratingly it can make it feel like you're forced to lose because you didn't know the layout of the level. Giving players the ability to see where these objectives will be would give them the power to feel like they can plan the level out in advance, while also opening you as the designer up to be able to create even more challenging levels. This would also remove the frustration of dying and resetting the whole level, as the player could only have themselves to blame for not planning out the level in advance.
I think you showcase strong level design with just a few simple mechanics. With a few tweaks to improve the player experience, I think this will feel even better to play. Great job!