I disagreed with you saying that they regressed with the E-series because the Germans never adopted a MBT design strategy. The Tiger filled the Heavy tank role and was originally only envisioned to be transported directly to where it would fight, break through enemy defenses and allow the medium tanks to then exploit the breakout. The Tiger's flaws only really began to show when Germany was forced into a fighting retreat and they were unable to dictate where the Tiger could be deployed - they essentially lost the initiative and the Tiger was ill suited to redeploy quickly on it's own while defending in order to counterattack. The Panther was simply the next iteration of a German medium tank design, which went way overweight as did most designs once Hitler started sticking his nose into things. And it never even fully replaced the Panzer IV, which IMHO was probably one of the war's unsung heroes considering what that model was originally designed for. As for the E series, that wasn't a regression away from a MBT concept since the Germans never grasped that possibility, the E series was simply an attempt to simplify the already existing light/medium/heavy tank design paradigm and reduce the number of individual parts that they had to manufacture (as they weren't in a position to enjoy making every part on every AFV unique unto itself).The German high commanddefinitelydid not understand the war of attrition early enough to make a difference but their design bureaus never really 'got it' either. Granted, most of the design concepts were solicited by Hitler's wacky idea sessions, I will give you that. But they kept making the wrong decisions. It would be an easier leap of faith to say they were too stressed if they hadn't kept misapplying resources to build heavy tanks despite making huge advancements with the Panther. They knew exactly what the M4s and T-34s were doing to them (hence the design of the Panther), yet they kept trying to fix it with more armor instead of improving the better performing and more easily manufactured tank. First by rushing the Tiger into service, the Tiger 2, the Maus, trying to up-armor the Panther into a shitty Tiger II and finally with the E-series designs that, once again, placed the wrong emphasis by basing the common components on the needs of the heavy tank production. That they 'reluctantly' kept producing the Panther because they basically had no resources to turn it into a heavy tank is pretty telling that they were just plain playing a different game. Which is odd, because they largely created the game in the first place...
Most of their late war armored vehicle design ran parallel to each other. The Tiger was improved upon with the Tiger 2, Panzer III/IV with the Panther (and later intended to see the improved Panther designs), and you also saw that with a move from the Jagdpanzer IV towards the Jagdpanthers/Jagdtigers. It's undeniable that the Panther was one of the precursors of MBT design, but I don't think anyone really put all of the pieces together until after the war ended. Both the Russians and the Germans spent no small effort in trying to outdo each other in regards to tonnage, armor and firepower but the Russians were the ones that could afford to simply throw massively overweight/overgunned AFVs into the mix in large numbers w/o hurting their overall production. Sure, the Germans could have made more numerous lighter vehicles along the lines of the Panther, but the Panther wasn't invincible and the Germans had a hard time in the late war period even being able to field enough crews to man the tanks they had anyways (nevermind being able to supply enough ammo and fuel for that many tanks).