You will never have a second panther
You have no grens, you have no MGs, you have no pios. You'll be a manpower-starved player twisted by my calliopes into a crude mockery of a perfect unit composition.
All the "comeback chances" you'll get are impractical and half-hearted. Behind your back your commander mocks you. your announcer is ashamed of you, your "grens" tremble at the thought of a rocket barrage.I can't be fucked to coh-ify the rest of the copypasta
Panthers are actually a bad choice to counter this, the win condition is to destroy the AT guns, if you decide to suicide rush the calliopes then you'll actually lose the trade since calliopes are actually very cheap, and the panther is worse against infantry than the P4 so it will give you less map control.
The counter unit is the brummbar since it hard counters handheld AT and ATguns, an ostwind or Pwefer might help decrew the guns in theory but people who went for those in my games didn't manage.
OKW is in a somewhat better situation since the rakkettenwerfers are easier to save and their HQs can tank several volleys of calliopes, often attracting AT guns out of position to deal with them.
Outside of that they have no useful non doctrinal unit, unless you get lucky with a walking stuka barrage.
The KT is obviously good but by the time it arrives you'll have 0 map control and fewer squads.
flammpanzer hetzer should be very good though.
The actual best way to counter this strat is to play well: get early wipes and don't let the allies win the early to mid game (which they are geared to do) but, up until the point where you'd normally get a tank out, USF plays exactly the same (RE spam is unusual but by no mean weak) so nothing stops them from not committing or get 2 calliopes into a jackson.