Profile of Serrith

You didn't even know how heavy cover worked until recently, stop talking lol

Ouch!

While ostensibly a "hybrid" unit that can tackle both vehicles and infantry depending on how you use it, the Su76 ends up getting shoehorned primarily into an AT role while its anti infantry qualities are only used incidentally. This is due to the fact that its actual anti infantry ability costs munitions and is already available on zis which arrive earlier and you will almost always have.

So how to fix it? I don't think making the current ability free would be a good idea, especially given that the zis version actually costs munitions and is on a less mobile chassis.

But I do think changing the ability and making it free might make it more desirable.
If the ability was a slow area denial barrage with long range akin to a howitzer (but of course with less potency per shot), and with a moderate cooldown that gets reduced incrementally with vet, you could make it free. Think of it like an rocket artillery "lite".

Of course you still have redundancy issues, why get an su76 free long range barrage when you could get a katy? Why take the su76 light AT when you could get an Su85? But it would make the su76 more of a true hybrid and a free barrage would make it far more desirable even if it's not as potent.

The churchill is squishier than the katyusha because the Katy stays behind the lines!

Churchill takes full damage from railway artillery while katyusha only takes 160, katyusha OP.

In my experience REngis take much more casualties, stock and vett'd, than Rifles and Penals even making use of cover at every opportunity, waiting to engage so they aren't automatically targetted etc.

This comes down to usage and not inherent durability. If you use rifles and penals at mid and long range, and RE at close range, then RE inherently will have to suffer more casualties since they have to close distance. This is completely separate from whether a unit of RE will die faster to continuous fire than a rifleman squad in a controlled environment.

Wouldn't anyone just rather the Jackson than anything else

I think the Jackson is generally going to be SAFER than the other TDs for most situations as while it doesnt have some of the strengths of the others (durability/cammo like jp4, self spotting and excellent rate of fire like su85, tulips and 200 alpha shots like firefly) it has very good mobility, reasonable accuracy on the move, good penetration, self repair and a turret. It just lacks real exploitable weaknesses unlike the other TDs.