I think the ISU works well because, yes as you said its long-range and it forces the other player to be aggressive, which works with guards defensiveness. It also works well (in theory) because in order to take the ISU out they have to dive, which is where the guards can plink away at their tanks and slow them down etc. That combo alone wont stop a good dive however.
ISU could also be good because this is a very manpower heavy strat and you might not be able to afford to spam t-34's.
Try experimenting with the Quad half-track behind guards/your line to stop too much blobbing.
Also Elephant can counter the ISU pretty easily and your guards wont be too effective vs that...
Having played a few more games after the one I posted I feel like we may have a solid, non-meta build here with a bit of refinement.
The games I lost were quite one sided, unfortunately. Timing the flamer clowncar can be tricky, because you want the car to hit the field asap around the time you hit 60 munis. It needs a somewhat significant impact to hold you over to the T70 and Guards phase of the build. In my most significant loss, I loaded up and tried to retake my cutoff with the car, finding that my opponent spent the munis I spent on a flamer on a teller. I dont even think the paint was dry on that poor car. After that significant blow, I struggled to get my T70 out, quite late, due to poor fuel and map control, and ran it straight into another teller around the middle of the map. Lol!
Suffice to say, this build really struggles with mines and playing from behind it seems (probably any build has a hard time catching up after big losses).
However when youre ahead with this build youre really ahead. I found that with good impact from the flamer and T70 leaves you with just enough munitions to get DPmgs on the three (3 seemed like the best number, before adding a 120mm mortar and going into T34/85s.) Guards and enough space to plant a demo on the main MG nest your opponent uses. Blow it as soon as he pops back in and you can ride the 85s to a smooth win with the button ability along with mark target.
It seems the hardest part is actually getting to the guards without too heavy of losses to keep going. The sniper really helps, so id suggest getting one after the car to keep you in the game. A very agressive Ostheer is very hard to deal with, especially with mines. (Ost players - build mines and delay weapon upgrades, you wont regret it!)