It's not FOSS, and it's been a few years since I used it, but I used to rely on Jota text editor. It was a very straightforward one, no bells and whistles, but that was kinda what I wanted from it. It didn't get unmanageable when using very large files, and if I recall correctly I think it also handled both Unix and Windows line endings, which mattered to me.

Not sure if it's still good, though. I don't do nearly as much weird stuff with my phone as I used to.