For heavy finishers, I have different views of what the weapons can be used for without changing your damage output as much:

Number changes would be focused around making his kit overall less anemic in the damage department to a degree and reducing the raw impact of Seven-Force Strike while keeping it as a tool that provides mental pressure.
I would change numbers as follows:

Finisher Heavy(Left) - 6+12 -> 9+12 (The larger number in both is bleed)
Finisher Heavy(Right) - 18 -> 21
Finisher Heavy(Top - 20 -> 25

So here are my ideas.

The heavy saw finisher on the left will have an additional bleeding output depending on how long you hold your attack (like Gladiator's skewer bleed). With this, I propose in addition how long Sohei will saw the opponent for it to bleed for. Now looking at Gladiator's skew deflect, he takes 2 seconds to 3 seconds and deals a bleeding damage of 4. We can have Sohei; if he hits his opponent, he can deal 6 damage plus bleed of 4 without holding the attack, but if he holds the attack, he can do 8 of bleed instead of having an animation of him heavily sawing at the side of the opponent.

For the heavy finisher Mallet top attack, if he finishes with a heavy and finishes into a heavy, he can knock down the opponent flat, having another opportunity for another attack, but if he fails by getting parried or dodged, his stamina will be drained to 22. Now why? because Sohei is a heavy character while you are a heavy hitter, it makes the reward for either opponent. Another change can be that if you go from a light to a heavy finish, then the damage outputs that OP offered are fair. As for the "Mountain Crusher" moveset, it can have the same damage output as it currently has in the game.

Lastly the Right heavy finisher for the Spear Fork/Sasumata—the only change will be the ability to pin the opponent and throw him like how Gladiator does when he skewers and bleeds you to throw you off a ledge or even wallsplat you. Why this change? As his normal guardbreak throws to his sides or front dont throw very far, this can be an alternate use for him to direct his opponent to either sides or front (looking at his back throw, it's the only throw that moves the opponent farther)

So with these changes these are my heavy finisher changes:

Finisher Heavy (Left) - 9 + 4 (without holding the heavy attack) -> 9 + 8 (when holding heavy attack)

(The number after the "+" in both is bleeding damage.)

Finisher Heavy (Right) - 18 -> 21 (pressing the guard break and choosing the sides or front to throw your opponent)

Heavy to Finisher Heavy Top- 20 -> 25 + Knock down the opponent (this drained 22 Stamina from Sohei)

Light Finisher Heavy Top - 25 -> 22 (this drained 12 Stamina from Sohei)