Few observations just playing pre-prestige:
The +1.5 damage upgrade seems to be bugged, and is being rounded down to always give +1 damage. Could also stand to be a few more damage upgrades in general; you cap out very early and then end up needing dozens of hits to kill pegs late-game.
The rate of fire number in the stats panel is potentially confusing/misleading: it says N/s (implying N shots per second) when the number represents delay, i.e. N seconds per shot (N S/shot?)
The shot delay upgrades are basically scaled backwards: the early ones are -0.1 which is a tiny ~2% rate of fire increase overall (going from 5.0 to 4.9 seconds per shot) and not even enough to get you an extra ball launch when rounds are short, while the last one is -.05 off of 1.35, ~58% rate of fire increase. Generally if you’re going to go with delay reductions instead of starting from frequency and taking the inverse, it’s best to start with higher values and gradually slim down, to preserve the proportionality of each upgrade (5.0 seconds -0.5 to 4.5 seconds for ~11% RoF increase at the start, 1.0 seconds -0.1 to 0.9 for another ~11% RoF increase by the end, as a napkin-math example)
On that subject, the “round duration” feels like it could be replaced with a simple “balls per round” attribute, given all the duration of the round really does is determine how many balls can be fired anyhow, just with additional complication with the Rate of Fire value.
The end-of-round popup obscures the multiplier half of the in-game scorecard. It should probably center on the game board instead of the screen as a whole.
As another random suggestion, you could have the multiplier slots replace missing pegs instead of disappearing if you’re worried about players tunneling to one multiplier and then spamming it. Though to be honest, the whole Balatro scoring system feels a little out of place to me.