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?
I dual boot and actually have a second NVME drive with Bazzite on it, but it's not visible under Windows.
It seems like it's just not possible to have games (on the F drive) accessible to both without problems? Bazzite says something about it being a bad idea to run games from an NTFS drive every time I mount it.
If you're dual-booting, there are some potential problems. The biggest danger comes from hibernation. Hibernating one OS and then booting the other OS and modifying the files (even incidentally just by mounting the filesystem), and then loading the hibernated OS will cause very bad things to happen. To the best of my knowledge, the Linux NTFS drivers refuses to operate on a filesystem which appears to be hibernated, but I do not know if the inverse is true.
Without knowing the exact text of the warning you're getting, I'm assuming it is a "this is not officially supported, don't blame us and don't send us bug reports" sort of deal. From what I gather, the most likely cause of problems is the potential for creating files with illegal names on the NTFS filesystem (filenames containing a ':' for instance). There is an unofficial guide on the Proton wiki about running Windows games in Proton which are stored on a shared NTFS partition which covers the relevant potential issues.
If the main risk is games not running correctly, I wouldn't lose any sleep.
This setup is not ideal, but neither is having 10TB of games turned into 20TB of games so you can duplicate them on separate filesystems. A compromise of some sort is required. In the longer run, I would aim to replace one of these NTFS filesystems with a Linux filesystem and install games there that you don't play on Windows any more.
Fortunately the D and E drives are Movies and TV, respectively. F is games. My plan is to run Linux 99% of the time, and I can probably keep the few games or random pieces of software I can only run under Windows on the windows C drive.
Basically I just want to have a Windows SSD as a backup.
Would it be better to reformat the D and E drives as exFAT if I can cycle the content through another drive or something? I'd like to be able to access those two drives under both Bazzite and Windows.
I think exFAT would be a step down from NTFS. NTFS is a journaling filesystem which makes it a lot more robust against data corruption than exFAT. It also has a number of more advanced features (for a 30 year old filesystem) like symlinks, transparent compression, and sparse files. NTFS is also less prone to fragmentation than the FAT family of filesystems. On the other hand, exFAT doesn't eliminate any of the potential compatibility problems (like the potential to create a file named "C:"). The only thing exFAT has going for it is that Microsoft has actually published a specification for it, but in practice NTFS has been reverse-engineered enough to have multiple reliable third-party implementations.
If you need Windows and Linux to access the same filesystem, NTFS is the least bad option (the best option is probably setting up a NAS for this amount of data). Another option is to install third-party ext2/3/4 or btrfs drivers on Windows, but last time I looked into this, they weren't very robust.
Ideally yes.
If you can't then it will still all work most of the time but occasionally the drives will unmount from linux with block errors etc and will not work until you boot windows and run chkdsk. You can use ntfsfix on linux but it doesn't always work.
Imo it's best to leave D and E as NTFS. Maybe down the line you'll end up with some swap space (maybe after you get that SSD for Windows), but for now it'll work. I'd focus on getting the C drive empty as you can using the spare space in your other drives, then use it for Linux with a 200GB partition at the end of the drive for Windows.
I can't advise re: games.
Would an overlay filesystem work? Its been a long time since I looked at it (and it was for very different purposes) but if you can get NTFS game drive mounted as a read only layer of an overlay fs and then idk some ext4 as the writeable layer, you can prevent writing data to the NTFS drive and making windows sad with illegal filenames, while also getting to like, yk, save your savegames and whatnot.
Might make loading times pretty long tho. Idk what the overhead is for an overlay fs.
This is true, writing to a hibernated system is not a thing you can unknowingly do.
I've never hibernated on Linux but Windows is basically incapable of mounting any UNIX filesystem, so I can't see that ever happening.
Yes, but if the NFTS was mounted by Linux when it hibernated, suddenly this becomes a possibility.