Hi :) thanks for the feedback and the nice comment overall!
About some of your suggestions:

"continously clicking the mouse button to shoot can be annoying" -> true, however I do not believe that auto fire would be the best solution. The game has only two inputs, aiming and shooting, so if auto fire is implemented, there will only be one input left, aiming, which could make the game a bit boring to play. So what I could do is implement different weapons, as you also suggested, which would make the game less boring and the shooting pace less repetitive, with possibly lasers that do not require any input, or semi automatic that have some auto fire, maybe.

"perhaps another way to get around the level and breaking boxes other than shooting (maybe headbutting?)" -> this one was suggested by one of my colleagues who tested the game before the jam deadline. I didn't do it for the jam because I wanted the crates to be an obstacle challenging the player shooting direction, since he has to face the crates to shoot them, pushing him the other way where he does not want to go, so he would need to stuck himself in some corner or something, while adding the ability of destroying crates would have make it easier in my opinion. This is still a great idea, especially for an infinite mode, else as a powerup, or just for juiciness purpose and speedrunning :)

"this type of game fits perfectly with a procedural level generation" -> could work, but it definitely depends on many other design choices (typically, different weapons, breaking the crates by colliding, etc.). I thought of implementing random generation for the jam and make it an infinite runner, but I prefered creating a rather small adventure the player could play in a few minutes, which I felt more confident with. That's also why I added coins to collect, to add some replay values in case some people would like the game enough to want to replay it, though it is not procedurally generated! Also added the timer display for an upcoming version of the game with some improvements and bug fixing, which could also give another reason the play again (timer was already displayed at the end, but seeing it during a run adds something I believe)

Otherwise, thanks a lot for taking the time to give your feedback ! I don't think I'll work on a game after posting a slightly improved version, to be honest I'm proud of the game as a jam entry, but I don't feel like taking it further. But if I do, I'll make sure to consider your suggestions !

Thank you :)
I get your point, maybe I could replace them with some physic objects just to push a little bit the player but not damage him for it to be less punishing ? Not sure it would help since your complaint is about the hardness of controlling Lenny, but there may be something to do !

Thank you very much, that's a nice comment :) And well, I hope it will be the jam winner ! :D
You're right about the sound design, which I'm really not proud of. Repetitive sounds like shoot and coins might quickly be annoying and I think both were poorly made, so I'll change them!

Thank you :) the bullets pitch is already randomized a little bit, but probably not enough :D
I'm not proud of the sound design of the game, I'll work on that a little bit more !

Oh man, didn't see it! Thanks for pointing it out to me :D

Thank you very much :)
Not sure about the auto fire since my game already has very few inputs so it might get a bit boring. However you're totally right about it being tiring, so there are definitely things I could have done about it ! Like adding lasers or stopping the lava while on checkpoints, already done this one for a small patch even though it's not the biggest improvement about the issue :p