How to set pin speed/alternate function in STM32 Arduino (PlatformIO)

You can use the STM32 HAL (STM32CubeMX) even when using Arduino as a framework for STM32 boards in PlatformIO:

GPIO_InitTypeDef pinInit = {
  .Pin = GPIO_PIN_8,
  .Mode = GPIO_MODE_AF_PP,
  .Pull = GPIO_NOPULL,
  .Speed = GPIO_SPEED_FREQ_VERY_HIGH,
  .Alternate = GPIO_AF1_TIM1
};
HAL_GPIO_Init(GPIOA, &pinInit);

Use

#include <stm32f4xx_hal_gpio.h>

in order to include the HAL functions.