There's a point you hit where the chests are moving so fast I gave up trying to follow it, haha. It was fun getting to that point. Stocking up on as many + extra heart chances along-side being able to re-roll without a cost (as far as I could tell) really let you make a broken build, which is fun for a small game like this. I am biasedly in favour of the theme as well haha.
If you did want to push balance a bit, I'd imagine adding an escalating cost to the re-rolls + forcing the player to open at least x% of chests (where x is say, 1/4 of the number of non-mimic chests, rounded down) might force players to be a little less "I'm going to re-roll until I have more hearts showing up then mimics" as I was. But like I said, the broken-ness was fun.
All up, great work!