In terms of reliability many of the tanks the Germans are confronting are equally or as unreliable. So in that sense I think the idea of improving reliability is hindsight; put it another way if there was a room full of German tank experts arguing about next steps, I wouldn't expect a significant proportion of the room passionately extolling the virtues of reliability. For as long as the German approach remains artisan-like hand tooling looking at quality, I think reliability issues would be accepted as a necessary trade off.
Well yeah but a lot of the reliability issues in OTL were acceptable to the Germans because their doctrine was working. Blitz through, surround and decimate your enemy, occupy the field of battle and move on, and then the clean up crew comes in and fixes the busted down tanks, those that can be repaired from battle damage, or those that can be scavenged for spare parts. When the Germans went on the defensive on the Eastern Front or later withdrawing this doctrine caused some serious issues to the Germans in available tanks.
ITTL this isn't the case. Here the Germans haven't been able to successfully pull off their doctrine and while in most cases they've retained the field of battle there is enough cases where they haven't. This is going to impact the number of tank available for combat. I'm not saying they are going to be pushing for a tank that never breaks down but an increase in reliability (say a 10~20% increase in reliability) over the older models during the testing and vetting phase.
Speeding up the long-barrel Mk IV would surely be the most cost effective first step. OTL as I understand it the 75mm Mk IV turns up around 1942, with some playing around with a 50mm in 1941. ITTL the 50mm upgrade might be deemed sufficient to meet what was being encountered on the Western Front, but whether or not they would have wanted to take the next step is also an option. The Char mounted a 75mm and this may well contribute to the development of SK's 'mini-Tiger'; a tank that can take on heavy enemy battle tanks where met.
I think the 50mm was a stop gap as the factories were not ready to mass produce 75mm tank barrels in the quantity needed, but you might be able to play with those dates a bit if needed.
I agree the 75mm Char (if available in enough numbers to make an impact) would likely instigate a design of a 'mini-Tiger' as we are calling it. It makes sense from a German perspective to do something now, but if overdone it might have long range difficulties if they adopt this their main tank. Cause it is going to be slow and certainly not very useful outside of attacking cities in the East.
Such an evolution would see the Mk III take on the role of the light tank, superseding the Mk 1 and Mk II; the Mk IV continues its role as an infantry tank, with a stop-gap off shoot to take on enemy armour; and the Mk IV comes in as the heavy hitter. I don't really see a Panther emerging, with its wide track, large road wheels and heavily sloped armor, unless its an act of German tank design genius.
The TD angle is a good point and worth a look; I hadn't traced its parallel development yet.
A good example of OTL German WWII TD:
The Jagdpanzer 38(t) - Hetzer
A Panther is too advanced a tank for right now (at best early '44) but with the lessons learned in France you'd likely see a more reliable Panther analog a bit earlier and available in numbers on the Eastern Front (assuming your TL even has one).
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An aside issue of this is the availability of experienced German pilots. My guess is most that are shot down over Allied territory are likely being sent to England and when (or if) Germany occupies France are not going to be freed. This might instigate a rotation change in the German air doctrine that will see less uber-aces but better overall training (through those experienced pilots teaching new ones) for German pilots. Instead of the 'fight till you die' doctrine of OTL they may adopt the US's model of where they go on so many missions and then are rotated back to train new pilots.
Just my two cents.
