You're clearly doing everything right—this is a common ESP32 upload issue. Since the Core2 boots and shows output in Serial Monitor, the USB connection works, but upload mode isn't triggering.
Here’s what to try (if you haven’t already):
1. Hold Boot Button (if available): While uploading, hold the reset/boot button until upload starts. Some Core2s need manual bootloader mode.
2. Try 115200 Baud: You've tried this—stick with it; higher speeds can fail on some USB ports.
3. Check CP210x or CH340 Driver: Reinstall the correct USB-to-serial driver for the Core2.
If none of that helps, your Core2’s USB-UART chip might be defective. Exchange it through M5Stack support.