Right now I've completed every single achievement, and I'm only 9 entries away from a full cakiary. All I'll say is that the game is incredibly fun, it manages a very nice balance between fetish and gameplay, though at further stages it definitely suffers due to how much of a bullet hell it becomes.
Biggest gripe is filling up the cakiary, powerups between stages in boss rush, or a sort of "check" system where if you have a powerup the game will spawn the boss you're missing (or one of, if you're missing more than one).
Gamemode problems would be that I see little reason to go and play in anything except standard, mixed mode or boss buffet. The other game modes are fun, but mostly at a surface level. Simple taste takes away all the fun cakes, watch your weight makes the main aspect of the game a detriment rather than something fun to look at, rainbow cake.... ok this one's fun if you're up for some heavy rng, speed eating feels a little too strong once you get to anything past 16 since you're not exactly getting stronger, and don't have nearly enough GP% generation to stay at the bigger sizes, and cheese it is just a fun distraction but nothing more althought I think that's intentional.
Other gripes would definitely be related to a few characters:
Pump Punk is insanely fun, and is probably my fave design-wise, but locking movement while damaging an enemy is very much the worst trade off in the game, especially at bigger sizes where you're basically a sitting duck while the next pump of damage slooooowly trickles up.
Massive Martha doesn't do nearly enough damage to justify how abhorrently slow the time between her shots is at bigger sizes, not to mention that this also makes the Martha-mode of Shifting Sugar's special transformation basically useless, as even though you're invincible you still can't one shot any of the non-standard cakes. Not to mention that you can get at most 4 shots out before you change to Bella-mode.
Bouncy Bella's clench mechanic definitely comes into play very much at higher levels, but having to give up a GP charge to initiate the mode makes it weird, as you've likely already averted the danger by using the burp. On the rare occassions this isn't the case and you burp, and are still in danger, then 9/10 times the clench wasn't really going to help you either.
Anyways, I'm still going to go on to finish the cakiary, and I can't wait for the campaign mode. Excellent work overall and best of luck to the whole team!