Mouth movement: It is possible to run along the inside ridge of some teeth. This gives you a much better line than running on the outside, but is harder and isn't possible with every cycle.

Mouth skip (Any% Only): If you press Escape, R, and Enter to kill yourself during the opening cutscene and then wait a second a press skip there is a chance you will spawn on top of the mouth. You can then run across the top and jump down to save time over running around the teeth. Shoutouts to Zir0nic and supercrazestar for showing me how this works.

Stomach skip (Major Glitches Only): Using the same setup as mouth skip there is a tiny chance you spawn in the stomach. Shoutouts to Zir0nic and supercrazestar for showing me how this works again.

Spawn in the lobby: Using the same setup as mouth skip there is a chance you will spawn in the lobby with the timer running. Not useful to my knowledge, but could maybe be part of some skip.

Esophagus double checkpoint: Using the gamepasses speed booster it is possible to run through the checkpoint at the top of the esophagus and fall down to the one at the bottom before the cutscene plays. UPDATE: I've discovered it is more consistent if you jump onto the first checkpoint and buffer a second jump.

Cancelling fall stun: If you hold the jump button until you land you won't get stunned when you fall from a great height (I think this is how this works. Not 100% on it though).

Avoiding stomach cutscene: When going from the esophagus to the stomach you can avoid the cutscene trigger by falling off to the right instead of walking forward.

Walking on acid: In the stomach there are points where you should have to wait for the platforms to rise out of the stomach acid. However, by holding the jump button when you land on the acid you can jump off without taking damage.

These are all the tricks I've found that could be potentially useful in a speedrun. If you think I should add anything just let me know :)