I enjoyed the game, nice mix of idle and active progress that both felt viable. A little bit grindy at the start but not too much... to be honest I'm not sure if this game would play worse without grind, because that seems to imply that there's a specific way to complete each challenge, whereas the current approach lets the player grind their way through rather than get stuck.
What might be a nice improvement is an interactive difficulty indicator before starting each challenge. Just a coarse-grained "impossible / hard / easy / trivial" based on static multipliers so players don't get stuck trying to finesse a level that really requires a higher multiplier rather than better timing.