Definitely gotta say that this game has a lot of potential and I am very excited to see it further develop into a full fledged experience, however likely that will be!
I think that your team absolutely nailed the presentation of a cartoony, fresh look with a stellar soundtrack on top to make the levels pop even just that little bit further. It gives me very huge nostalgia vibes and awakes memories of sitting at my grandpa's PC, clicking away to flash game to animation without an account. The game is an instant breath of fresh air for the rhythm gerne which I haven't seen this well executed since the Parappa Series.
One thing I would definitely like to point out is the massive difficulty spike from Week 1 to 2. The patterns become very unpredictable and keeping up is even made more difficult by the pattern constantly switching around, which is why I had to retry about 30 times just for Song 1 on normal difficulty, and then it felt more like luck. Comparing Week 3 to 2, 3 is way more manageable and builds up from the difficulty that was present in Week 1 imo.
I get that balance is tricky to get right, but I feel like you already have a blueprint here on how to scale the difficulty up with each following song. Building up on that foundation may prove more rewarding in the long run.
Regardless, FNF was an absolute joy to go through and I'll be seeing myself having the title song on loop! Very excited to see further work on this game!