No, 20s reload would be insane. Everything about it is fine, except for when it derps on terrain. You dont want to be nuked once a minute.
And dual 34/85s can actually have a hard time with it, you just have to take advantage of faction design: Couple rakketens in buildings next to the Flaf HQ is an ideal reloading base for the S Tiger. You can quite happily pop at any tank or blob and retreat to safety.
I will argue with any tank. Since sturmtiger lost ability to track vehicles it hardly going to hit any moving target. Blobs? It made to kill blobs.
Even if t34-85s could not destroy sturmtiger with direct assault, without panther or jagdpzner (and going sturmtiger means no panther or jagdpanzer for a loooong time) to support OKW infantry, t34-85s will give sov player so much map control and inflict so much bleed on OKW player, that some (if there would be any) squad wipes with sturmtiger in return would be hardly worthy trade.
Sturmtiger is very niche unit, but it under performing even in his niche. It not worth purchase sturmtiger when stuka is both better, comes faster and less risky. Look at wehr sniper. Relic buffed his hp twice (and 4hp more than they needed to if you ask me), but buff hardly made him op. I believe sturmtiger needs same approach.
ISU have potential to wipe with every shot. Taking assumption whole shoot-drive back-reload-drive forward to shoot cycle for sturmtiger takes 40 seconds, ISU (well, if static) can land 4 shots form safe distance in the same amount of time. Also, in addition to its far superior range ISU have almost same hp as sturmtiger does and more armor.