FWIW I didn't have any trouble with the timing it's more a suggestion on trying to get really immersive experience (which almost assuredly does help if someone is struggling with timing so I think I good idea), not that you need to be sold on that idea.
I think confetti is great solution, it's very clear and obvious but animations could do it too (or likely both). Some other ideas (possibly in addition to) are giving the completion time, tracking those times and alerting for a new record; obviously more work but maybe you want to develop it/explore more ideas? A less obvious but reinforcing could be that there is a "next level" and if you lose, it's greyed out but if you win it's active and green/has some sparkle/shine effect. I think what you did of being able to select any level is 100% the right thing for a jam but in a bigger release having levels unlock could be the way to go.