I have always liked the concept and see them as a replacement for towed guns. Even in WWII towed and pack, infantry support, howitzers and guns, of around 75mm were still pretty common and towed anti tank guns became very common, following the war they pretty much disappeared, replaced with recoilless rifles if anything.
To have Stugs survive all you need is for these towed infantry support guns and AT guns (and their tow vehicles) to be replaced with casemate type Assault Guns / Tank Destroyers, either as platoon embedded in mechanised battalions, or battalions supporting Infantry Divisions. Armoured and Cavalry units and formations would use close support versions of what ever tank they operate, i.e. CS Infantry and Cruiser Tanks for the British Armoured Regiments and M8s or 105mm Shermans for the US Armoured battalions and Cavalry Regiments. I believe the 165mm L9 demolition gun was originally designed and intended as a replacement for the 95mm gun as the CS counterpart to the 17 (or perhaps 20) pdr. Perhaps this could be an option for a post war casemate assault gun.
Love the idea and looking forward to the result, you will probably finish long before I start my planned Australian post war Stug.