MPU such as STM32MP15x are much complex product than MCUs, and learning curve is usually longer if you start from MCU experience.
Bare-Metal on Cortex-A7 is not supported by ST SW deliveries nor Tools such as CubeIDE or CubeMX. Nevertheless, there might be 3rd party which could help you.
External DRAM is almost necessary due to low amount of embedded memory, but this might not be the major issue if you reuse existing design/configurations (ST EV1 or DK2 board, or use HW from any 3rd party SOM or SiP modules).
Notice that using dual core in a bare-metal SW will be complex to do (memory sharing, resource/load balancing, etc...).
Using Linux might allow you to benefit of a large SW ecosystem.
Notice also that from paper, Cortex-A7 (~3 Coremark/MHz per core using GCC) is not as powerful than Cortex-M7 (~5 Coremark/MHz using IAR), except if your processing can benefit of NEON accelerator or specific Cortex-A optimizations.
Looking at wiki and the following posts and Application Notes could help you:
https://community.st.com/s/question/0D50X0000B06m50SQA/porting-to-stm32mp1-ethernet-and-other-peripherals
https://community.st.com/s/question/0D53W00000GYk3RSAT/how-to-manage-usb-with-a-baremetal-application-
AN5253 and AN5031
Official partners for STM32MP1 series.
Regards.
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