Wow, thanks a lot for taking the time to write a detailed reply, I really appreciate this.
And you are spot on with the crackling sound and performance going together. I first increased the buffer in pygame.mixer.pre_init to 2048 instead of 512. It did improve the sound but not completely. And then I spent a lot of time measuring the FPS and the time spent in almost each function to better understand where the "work" was being done... With regards to the sound, I think in there is still one particular screen where you can hear a subtle "hicup" the first time you display it, but the sound issue went away as I fixed the under-performances in most places. I know it's not perfect yet, but I think (I hope) it's good enough for now.
However, I had (and still have) plenty of ideas for the evolution of the game, and I have to say that this "webification job" dampened my enthusiasm... Now at least I'm really getting that a browser is not behaving like a desktop 😅
Thank you about the warning for the canvas resolution, I think it's really key.
I agree about the fact that the friction moved going from desktop to Web. And I wish that the loading was a little faster (though I also tried to squeeze this down to some extent). I'm using a template file to control at least the colors of the background and text displayed. I definitely could do better with the loading screen to make sure potential players don't go away before it's ready... but I think it's not so easy to control what is displayed very early on.
In any case, thank you again the explanation. It's a relief actually first to understand what happened and second to know that other dev experienced similar difficulty.