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?
not really a super easy way, you seem to have a ton stuff, i just uninstalled everything on windows in steam and then installed it in linux. each game has different ways of backing up a save, so be careful about deleting any save files
"just re-download 10 TB" is just a wild thing to suggest users do. Like, I get that this is a really niche case and most users aren't like me but it's wild that I will have to shuffle data this way just to keep my Plex library and stuff.
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.
It's copies of movies and tv shows for Plex. Yes, I'm worried about it but I can't afford the amount of storage needed to have backups of media available on my private tracker.
At this point your only options are the filesystem shuffle for a billion hours or using ntfs on linux where it might just corrupt your data one day silently or break in a way that needs windows to fix it. Or continue using windows.
I haven’t used ntfs on linux in a long time. Hopefully its better now then when i last tried but it can be afaik a bit nefarious where it works great until it doesn't
They are describing a scenario where enough of the data is steam games that deleting them would create the swap space needed to get started shuffling stuff around in which case it wouldn't necessarily be that wild... you would be avoiding a lot of the headaches that redownloading other stuff would have; high reliability of the files - nothing would be lost, fast downloads, wouldn't be much labor to que them up, you probably won't need all your games right away... your mileage will vary but it's not the worst suggestion for some people who might be reading the thread in a similar situation.
I ended up deleting all the games, and I'll reinstall those as I need under bazzite yeah.
Leaving the 10 TB of video alone ecause it seems NTFS will be Fine™ for just video files at least for now.