What can counter a decently/even marginally supported JT? Any advice would be very appreciated.
pretty much nothing. vetted OKW units give so much sight, this makes flanking attacks with medium tanks extremely rough. Your only hope is indirect fire spam, or direclty attack the JTs with priests and use the moment when the JT is damaged and needs to re-locate for a flanking attack.
You need to pray that the axis player is very sloppy and loses the majority of his almost immortal vet 3-5 inf to your indirect fire. Then you can hit their heavy tanks!
But if the OKW player has half a brain, it's usually gg wp.