The initial issue with the Panther D and the Tiger I was they were rushed into service and experienced break downs in the Russian front. The Panther A, G & F were vast improvements, I have yet to find why Panther model D was the first production and the second was A. The Tiger I A also had teething issues and Tiger I E was a greater improvement. Both panzers could defeat anything the allies fielded against them and form a distance. Keep in mind to mount heaver guns meant slower panzer models.
The 88mm assault gun Panzerjager Elephant (Ferdinand named after it designer) was built on unused Porsche Tiger I chassis which competed with the selected production Henschel Tiger I model. The Porsche Elephant lacked a machine gun for close fighting and was first seen during the battle of Kursk. It also served in Italy with a machine gun installed for the radio operator.
The Sturmgeschutz III & IV were built in the respective former tank chassis kept in production as tank hunters in separate units and were also replacements for Panzer IV units lost in combat. The Panzer IV Jagdpanzer was a pure tank hunter and both models had effective 75 mm cannon. The Czech Skoda built Hetzer with a 75 mm cannon was also a PITA for the American Sherman tank as it small profile and size could easily ambush allies tanks.
There were many other German variations of tank hunters like the Hunting Panther and Hunting Tiger II and smaller Martins series mounting 75 & captured 76 mm guns. Not much went to waste in the German Army.