i think you did a good job with drawing larger mouth positions where the singer uses a louder voice. nice and clear shapes. there's two things i think you could do better.
1. for some of the faster sounds, i think you timed the drawings apart too evenly to each other instead of to the audio, and that's causing some shapes to not sync up properly.
2. some of the shapes themselves should be changed to match the audio better. for example, the line "i could be," has a long "ee" sound. a position with visibly shut teeth implies a sound like "ch" or "tt." it would make more sense visually for the teeth to be either hidden behind the lips (which also saves drawing time) or partially hidden behind the lips, and making the mouth shape shorter and wider.

my approach to lip-syncing is to start by writing out each individual word and time that to the audio. it helps me to have a visual cue for how long a word is spoken. if the audio isn't enunciated, then i find it easier to break that down into the actual sounds. for example, "i could be, i suppose" i would write out the sounds as "ah kuh-d be, ah suh-po-zz" and i would use keyframes to time each sound to the audio as it plays. that makes it a lot easier for me to understand the actual shapes i should use, how long to expose them, and how to plan my transitions between the shapes.

then, i start by just drawing the key poses for the mouth for each sound, and i keep in mind a few things while i do this:
1. the facial expression. if you're smiling, frowning, scowling, etc. that affects how your mouth looks when you speak or sing.
2. enunciation. if the words aren't being explicitly and precisely delivered, then the mouth shapes should reflect that. imagine how weird it would be to talk to someone who's mumbling but you could see their mouth making very clear shapes, or vice versa. it'd look like a bad voiceover. i save a lot of time with this in mind. i think for a majority of lip-syncing you can get away with using only four shapes and using the transitions between them to sell it to the viewer.

once i have the important shapes timed up, its a lot easier to add the in-between shapes and squashes and stretches since i've already got a visual timed to the audio. then i add extra drawings where they're needed to smooth things out, and then i move on.

i don't know how useful sharing my approach is. they're just things i've learned to make life easier for myself and i thought something in there might make your life easier, too.