Update,
still haven't committed to buying another tablet. The first flight mentioned above with the iPad mini, it was sitting on the passenger seat or my knee as I had previously done with the iPad Pro.
For last weeks flight to Mt Gambier, I used a Ram mount and it lasted fine, both there are back.
I positioned the Ram mount on my left, attached to the canopy with the suction mount, just next to the vent and had the vent slightly open.
I think the combination of vertical orientation (less direct sun on the screen) and airflow obviously helps, both days it was only 21deg at ground level, and once we got above the cloud we actually had the cabin heat on, so not a direct comparison to the first effort.
I'm still going to buy another tablet in the near future as the borrowed mini is only wifi, no GPS or network connection.