I think this might be due to the Atmel->Microchip transition
Are there any plans for a successor?
Does the "Due form-factor" have a special name?
You may try STM32 chips, they got much more variants than arduino.
I assume there's no successor or replacement for the Due
I stumbled upon the "Status: Not Recommended for new designs"-notice along with the fact that this MCU is categorized as "Legacy Product"see: https://www.microchip.com/wwwproducts/en/ATSAM3X8E