Hi there, I'm Matt Sergej and can develop your Bash script for you (24 years a webmaster, started with Linux system administration, graduated and scripting multimedia for 11 years).
Now for the script here are my notes per your requirements, assuming ffmpeg library usage:
1. Script must identify a subtitle track with certain characteristics and burn that track permanently into video. - What are these characteristics? "Burning" the text at scripted position and between start and end time is possible. But are you asking for the independent subtitles - they can be in the file but not burned in the video e.g. this is a feature of the player to show them (like VLC). Burning "hard" requires encoding of video = losing quality.
2. Primary/default audio track must be changed to preferred audio language. - I believe this can be done.
3. Output video must be lowered to a given bitrate if necessary. - OK, if bitrate is possible to read and is higher than requested.
4. Output files/directiories creating/renaming. - No issues.
5. Script must have a system to avoid duplicate processing. - proposing a simple text files for a "queue". On starting a script all video file paths are written there. After each processed video its path is removed. If script finishes, file is removed, if interrupted and rerun, checks for the file and continue/run.
6. Deleting files after X days - if not a cron scheduled job, then a parameter set at calling script. Let's discuss this.
Best Regards,
Matt