That is the most basic thing that should work. Can you try a minimal firmware
#include <Arduino.h>
void setup() {
Serial.begin(115200);
}
void loop() {
static int i = 0;
i++;
Serial.println("Test " + String(i));
delay(1000);
}
? Maybe needs monitor_speed = 115200 in the platformio.ini. Is there no output?