TfL doesn't publish the extension fares for contactless outside the zones, but I thought that inside zones 1-9 it was simply the regular fare for the remaining zones, so with a Z1-5 travelcard you'd be charged a Z6 single. Maybe this has changed.
The fare for a Z6 single is £2 / £2.10 peak and the Elizabeth line Heathrow premium for non-Z1 is £5.20, so potentially you might be charged £7.20 / £7.30 peak.
If you really don't want to pay extra, after arriving at a Heathrow terminal station you could travel to and tap out at Hatton Cross (zones 5/6) on the Piccadilly line, and with a different contactless card double back on yourself by performing a free interchange between Hatton Cross and your desired Heathrow Terminal on the next Piccadilly line train. This would add roughly 30 minutes to your journey.
Never mind, if you use the Elizabeth line to arrive at a Heathrow terminal it appears you will have to tap out and you will be charged the journey to Heathrow no matter if continue to Hatton Cross.
To use this method take the Circle or Hammersmith & City line and change at Hammersmith, taking the Picaddily line to Hatton Cross avoiding zone 6, then tap out and back in with a different card as explained above.
Yes, each of the Heathrow Elizabeth and Heathrow tube stations are completely separate, otherwise it would not be possible to charge a premium.
If splitting at Hatton Cross there is no need to use a different card.
If taking the tube and not passing through Z1, splitting at Hatton Cross saves extremely little if anything.