How to burn the bootloader and upload sketches on ATmega328P U (specifically this model)?
IT WORKED, FINALLY. I followed this tutorial: https://www.electronics-lab.com/project/installing-the-arduino-bootloader-on-the-atmega328p-microcontroller/, followed exactly as it is explained, made the connections (nothing more, nothing less; I even removed the LED on pin 19 and its resistor) and to my surprise... the bootloader was actually loaded and the sketches were uploading just by using the FTDI. Thank you a lot for your help and information and I hope someday I'll pass this knowledge to someone who will be in the spot I was until now. Have a great time and love ya all