Hey, I'm glad this has finally released!
I got around and played the whole thing, didn't collect all 200 emblems in every stage, but I think I gathered enough for this post.
To begin with, the mod itself is really well made, it controls fluently and the characters are unique and balanced, and the aesthetics are amazing, I got nothing to say on that.
Now, gameplay wise, in SinglePlayer, I will say that it's fine most of the time, excepted in those parts that seem like they're expected to be played with a full 4 players due to the amount of enemies. But that's perfectly doable.
The level design in itself was good. Linear, but good, there's nothing wrong with it. What bothers me the most is the enemy placement, especially in Weather Factory Zone where you placed those Kracko a-la Mysic Realm to be as annoying as possible, and this enemy is a pain to deal with, even when you're in reach of it, in most cases I got around and killed it, I was hurt by the tiny... bomb seed it let out 1 frame ago. I also didn't really like that section where you have to run from the C.H.R.O.M.Es in Mainframe Metropolis, especially as Tails since he cannot seem to reach the tiny platform in this huge gap without having to fly, which significantly slows him down and often gets him hit down into the pit, the only way to avoid this is to have a good headstart on the enemies behind you from the start and to keep it, but even then, not getting hit after that gap is a challenge due to the walls and the huge speed drop flying inflicts on you.
The bosses now are roughly "okay" in singleplayer. Mainframe Metropolis' appears to have a glitch though,
Tails can go over the invisible wall in front of the camera and go hurt the boss himself, which is quite funny if you want my opinion ;P
The boss itself now is fine. It's just a bit frustrating how at some points, none of the balls are actually aimed towards you, and it's kinda random when you manage to hit one. (and then it still has to land a hit)
Frozen Factory's has a bit too much health for SP in my opinion. The fight is not hard, it's just longer than it needs to be for a person, and it makes the only 3 attacks it constantly uses become old really quick. Also Sonic seems to often get stuck in the oil and die there without you being able to do anything about it pretty often, it's pretty frustrating considering how easy you can be knocked down there.
Weather Factory's boss was... a waiting game. The first phase is just 1 minute of waiting for the balls to go away and jumping on it. The second phase is pretty much trying to hit the boss without getting hit yourself in the 30 seconds time limit, which is pretty hard considering the area's darkness doesn't help you spot the shadows easily, and the time limit makes it all the more stressing. I think it would be a much better boss if the first phase actually TRIED to attack you. The second phase is probably meant to be that chaotic struggle, either way I found very well done how the arena slowly falls to the ocean, kudos for you on that!
Now, the multiplayer.
I have to say that, on paper, the concept of replicating Mario 3D world's gameplay with this sounds great! But in practice, it doesn't go very well. As stated in the OP, more than 4 players make the game roughly unplayable, and even with 4 players, it's still pretty hard not to turn into a bubble every 20 seconds. I get that it's trying to mimic 3D world where everyone had to stay grouped, but it's because it was local multiplayer on one screen, here, we can have independant cameras that are more focused on what individual players are doing,
because the current camera often gets stuck in really awkward positions and nobody can really do anything. I'm not saying to remove the "staying grouped together" aspect, but just making the camera a bit more client friendly would make the whole mod a lot better, along with maybe raising the distance from other players before you actually turn into a bubble, since you can't really communicate your randok netplay buddies where to go outside of the chat, which takes more time than everyone turning into a bubble and getting a game over.
I think that, in fact, the current camera would only work well in a group of 2~3 people in a Discord call, or in actual local multiplayer. (but nobody plays that so...)
Overall, my final verdict is that it was a fun experience, but easily becomes pretty frustrating in netplays when everyone tries doing something different and all end up in a bubble. But aside of that, when you get to play the mod with 1 or 2 friend(s) that you can communicate with, it's really a great time, and I'll definetly be looking forward to whatever update comes out in 3 years or so :P
also what is the music in Ancient Waterway? I got that feeling I know it but I can't seem to remember it at all!