Thanks for the feedback! Trust me, I would have loved to implement many more effects, visual and sound feedbacks... I just had absolutely no time to do so (worked only for the last 3 days of the jam on my game...). It would definitely make the game look more polished than it is right now, as well as make it more fun to play. I mainly focused on visual and sound feedbacks that would communicate to players what was going on on screen, but had to leave for a later time most of the ones that would make the game "feel better".
And overall adding visual effects is just something that takes me quite a long time and a lot of trials to get right. For example, I actually drew an animation for the lightning attack. I implemented it and it just... didn't look "good/right" for the game, so I scrapped most of it (and obviously didn't have time to revisit it). And as you said, the character animation to indicate that the spell is ready works, but it's not quite good/striking/eye-catching enough as it is (some sound effect would also have been good). Some screenshake when the wizard is hurt, or when a knight dies would also have been good, and honestly I could go on and on on all the things I could add to make the game look/feel better. So yeah, absolutely agree with your points!
Ahah yeah, I was actually surprised at the number of players who chose to go back in alone immediately after losing!