these are great suggestions, as for the boost mode part- we're still trying to find a way too keep it on his moveset, as of now my idea is to make it so you can't lose boost mode while midair and the stomp turns into a bounce (sonic advance 2 downward trick) that maintains boost tooHi Hi
I've seen some people recommend punching as an ability, if you do go for that as an ability I recommend basing it off of SA2 version where the punches actually helped you keep your momentum when making tight turns if you've seen SpeedRunning of his stages people throw the punches out to keep his speed up.
You also said you had dig planned so that will probably bring the drill dive with it but I think a unique spin on that would be that when you dig into the ground when you are at high-speeds you store your built up momentum and rocket out of the ground at maybe an acute angle I feel like that would compliment knuckles pretty well and fit well within srb2's level design. You could make it so he can rocket out of walls too.
I would also go against a boost mode since knuckles is very stop and go unless the mode is made more flexible or is actually a mode change like activating speed shoes but that might over complicate things.
Advanced 3 also had few unique quirks, with the Knuckles and Tails Combo Knuckles would do a small uppercut then glide basically a double jump that would lead into a glide.
Sonic & Knuckles: would remove the glide for a headbutt but can't climb could add that as an optional way to get a burst of speed but losing the ability to cling to a wall when doing it as the trade off for greater speed.
Knuckles and Amy: gave Knuckles the ability to dash up walls, just like how Mecha Knuckles does it.
Knuckles and Cream: made it so he could homing attack enemies close to him during his glide, honestly don't know how useful the last two would be for fleshing out a move set but might as well list them all out.
I hope this is useful information that can help you in some way if no one in the Discord Server has brought up these in particular.
your right but he could have a moveset it would be cool