Flashing a bootloader on Ender 3 without Arduino
What you need to is called a ICSP or ISP: in-circuit serial programmer or in-system programmer, which excludes the USB to TTL device you own.

I've never used an ESP8266 as ICSP but it seems there are some resources out there reporting it is possible.
If you want to go the easiest way probably you want to buy an Arduino and follow the tons of tutorials out there, if you are looking to save some money then you might get around buying an ICSP like the very well known USBASP (just Google for that).