STM32H757I - EVAL BOARD to learn Embedded Systems
You don't have to be an expert in C. But it helps if you are, because sometimes in embedded systems things are done with slightly different mentality compared to how you would do it on PC. The C is still same, but you don't have a familiar OS running the code, as there is no OS at all, or there is some RTOS. Have a C reference at hand. But there is so many other things to learn too than just C. You have the IDE or other environment to write and build code, to download and run and debug the code running on MCU. And the MCU itself, at register level, or at the HAL level provided by manufacturer, or the electrical datasheet level, or assembly instruction level.

For reasons I said above, forget starting from registers. Install CubeIDE and start working. You should be able to compile, download and run a demo program within few minutes. It helps if you are familiar with Eclipse-based IDEs.

Not familiar, no comment.

Manufacturer has all the register level details in the MCU Reference Manual. You don't need any tutorials, the information is already there.