pwnedroot/RedditTool ? reverse-engineered prompt

Build me a simple Python tool that checks Reddit username availability in real time. I want it to generate usernames from letters and numbers, try them fast using multiple threads, and show a progress bar with an ETA so I can see how it is going. It should let me choose the username length and the character set, because I may want short names or a different mix of characters later.

When it finds an available username, save it automatically to a text file so I can review them afterward. Keep the script easy to run from the command line, and make sure it handles Reddit rate limits reasonably so it does not spam requests too aggressively. If you need to check current Reddit related behavior or Python library docs online, do that.