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.

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.