Questions about designing my own BLE-controlled LED device
The NRF52833 SoC is a 64 MHz ARM Cortex M4. You can program it directly without having to use another microcontroller. Nordic provides an SDK for it, with SoftDevices that implement the BLE protocol stack as well as other RF stacks. Unless there's a specific reason you want to have 2 microcontrollers in this system, I think you can ditch the ATmega. Even if you'd prefer to use an Arduino, because of your familiarity with it (understandable), you'd still have to program the nrf52 as well. So I think you should just do that.

If you want to use this project to work on PCB design, you could basically design your own dev board for this Nordic SoC, and then use it to flash the BLE stack onto the chip.