Finally ready to switch my final windows device over to linux, but I'm running into a problem. NTFS drives aren't a good idea under Linux, but my 4 NTFS drives are this full. I own zero extra hard drives, and do not want to spend $300 on another one. 
How can I bring these files into Bazzite WITHOUT just wiping everything and re-downloading 10+ terabytes of movies, shows, music, games, and other data I refuse to delete?
What you’re asking for isn’t simple from a technical perspective. Tools to rewrite a filesystem do exist but they’re risky as fuck so its not like a standard experience. The best option is to copy your data to another device then copy it back but also like do you like not back this shit up at all? Are you not worried if a drive dies and losing everything?
The only real option with your existing setup without buying anything is tiny ext4 partition copy resize ntfs and ext4 copy but you’re gonna be there for a while. If there is a single drive you can realistically wipe for this procedure to give you a bigger copy to space in ext4 then that will speed it up.
If I was doing this then I’d opt for robocopy to a drive big enough to store all the stuff i wanted to keep in a filesystem thats cross compatible then rclone it back.
Naturally windows linux and mac dont like each other and moving from any os to any other is quite the task if you arent willing to delete everything and start again.