I had a crack at a SPIFFS filesystem driver for lvgl a few months ago. It kinda worked as I could get an image to display properly, but it still crashed sometimes… so expect it to be buggy.
You might be able to use part of it for your own implementation or maybe someone can help find some of the remaining bugs. I have it on GitHub here https://github.com/fvanroie/hasp-lvgl/blob/0.1.0-dev/lib/lv_fs_if/lv_fs_spiffs.cpp
It’s been a while, but I want to re-investigate the driver too, as this question comes up regularly. Hopefully we can join forces and create a working reference driver for ESP32.