Depends on the data being stored. I don't know how large my complete Comixology library is but I'm sure it's in the TB and thus far, I don't think that's allowed on external storage on iOS. My non-Comixology manga collection is around 2TB.Can't imagine needing more than 256 with external media now working with ipad
Besides, while you can watch videos hands-free, having a flash drive or portable SSD protruding or dangling from the USB-C port is uncomfortable while reading.
I don't need to carry my entire library with me but I do like having a bunch of different reading choices available on hand. I need at least ~150GB for manga and another ~200GB for comics (favorites + reading list).