The E8 demolishes a P4J too, because it has more hitpoints and more armor. It's an excellent vehicle against all medium tanks.
But the game generally does not work that way. Tanks do not suddenly appear en masse in a vacuum. You'd be getting a Sherman out first while Axis get a P4(J) and they are going to cause bleed to your infantry while also having an advantage in the armor match up. Then you're getting a Jackson while they are getting a second P4, a TD or a Panther and you'll be down even more in AI bleed. With E8s you can just keep stacking them, dealing decent AI damage while also dominating the tank engagements until the Axis are forced to invest in a Panther or a heavy to deal with the E8s, at which point you'll have enough of them (2-3) to fight that off too. This is exactly the reason the 76mm was the dominating meta over the past months.
Demolishes is a rather strong word for the performance. Doesn't the E8 have 215 armor and P4J have 234?
Doesn't the Sherman have better AI so wouldn't a similarly competent player cause more bleed? A second Ost P4 would lose, P4J would be zoned out and the tank destroyers wouldn't cause any more bleed due to their horrific AI however I would expect the Panther to cause the scales to tip in favor of axis however due to cost I would have a time of opportunity to attack. But since you said 2-3 and included a heavy, 1 Sherman and 2 Jackson would demolish any Heavy while having smoke to retreat.
Also the 76mm had vastly superior ROF due to base stats plus VET along with 15 fuel cheaper cost which allowed and encouraged blobbing. The E8 doesn't have those perks.
Would you be against keeping everything as is but increasing the price of the Tank commander to say 70 muni while giving it the ability to call down a arty stike? Maybe something like the first Shell of the 240 artillery from Armor company. That way the commander has a way to deal with emplacements /PAK43 /Howitzer. A shared cooldown between all E8 to keep it balanced.