time gear
Eternity in an hour
No no no I don't think the levels need to be changed. It's only additive that I'm proposing. Adding an additional accessibility option for new players. As it is now, it is understood that vanilla Sonic solo is hardest. Tails mitigates that, but at a cost, because you're room skipping, and likely to get lost.
Sonic having recovery options (homing/double jump/wallkick) makes levels significantly easier than having one jump, and the Thokk.
Imagine this play style to be optional as Sonic
jump x2 --> homing attack or double jump
jump + dash --> thokk
dash --> dash or roll
--When you have a shield, jump + dash is the shield power instead of thokk.
--When you have 7 emeralds and 50 rings, jump + dash is how you activate.
--And I'd include the walljump, too.
Ultimately my reasoning is that Sonic should have the option to be more accessible, because he is the most fun to play, either way.
I don't really care for Boost, and I like light-speed dash and downward stomp okay but not enough to advocate for incorporating C1 and C2.
By just giving Sonic double jump and homing, you are giving the player more ability to adjust in mid-air, which helps mitigate the difficulty by A LOT.
It makes sense for the layout too, because Thokk is really "spin dash in midair."
It's true the game was not made with this in mind. But it works beautifully. And it should be incorporated to be offered as an option for new players.
And it doesn't impact anybody who enjoys Vanilla Sonic as it is. Once a player has played with the training wheels of homing attack, then they will enjoy playing with only the Thokk. But you START with easy, and then work your way up to HARD... and then once you can do it, it's very satisfying and rewarding. You don't want to start with hard and make your way down to easy.
Based just on my personal play, having the Modern training wheels has made my jumping as vanilla Sonic better simply because I have had more success experimenting with the physics in the game. So I don't think it really spoils any enjoyment, only encourages more play-throughs as harder characters like Amy.
Also, on the other hand, it's not a terrible thing if Sonic is the most fun character! Or even if he's the easiest! Makes it more fun for the additional characters to be the hard mode.
If Modern Sonic was implemented as his own character, the levels would have to be adjusted to acknowledge him unless he was literally the last thing you unlock or else it wouldn't feel fair that he doesn't get his own pathways while everyone else does, and that in fact he has the most freedom to go whichever way he wants out of everyone.
If his moveset was applied to Classic Sonic as an option, it would beg the question of why Classic Sonic has an optional alternative moveset and not anyone else.
On the topic of making Sonic more accessible however, you could have Amy be the "Hard mode" character in Sonic's place and tweak Sonic's moveset a bit to be more beginner friendly.
Something I have heard even before you mentioned it is giving Sonic the double jump. What might actually work well, though it would take time getting used to, is if while you are midair during a jump you have the option to do one of two things: Pressing spin during a jump lets you jump a second time, though this gets replaced by whatever shield ability you pick up until you lose the shield. Pressing jump while you are midair during a jump allows you to thok. When Sonic turns super, he gains the ability to combine these actions once per jump in whichever order he pleases, at the cost of losing the hover. This would overall be a nerf to Super Sonic, but would make him more satisfying to pull off skilled maneuverings with.
This would make Sonic more beginner friendly and have the added bonus of making Metal Sonic feel less like a Super Sonic clone since the hover would now be exclusive to him.
This would also leave the ability to have Modern Sonic proper as an unlockable for 100%, while still allowing Classic Sonic to fill the role of being more beginner friendly.