I have a Kobo n437, I swaped the internal sd card with a 32gb one and installed InkBox/Quill OS.
You'll need a bit of linux skills to use a bigger storage solution to its full potential. The project is currently on hiatus as they port the OS over to the pine64 e-reader.
Worth noting otherwise, currently pictures in epub formated books don't load (at least on mine don't know about other models) so youll want to convert them to pdf.
The process isn't hard or time consuming, but I understand, this sorta stuff isn't for everyone. I'm just a huge proponent of "this is my device it'll do as I say"