Not much testing, but this should work for using LittleFS with the Raspberry Pi Pico built in flash chip.

rpi_pico_littlefs.h

#pragma once

#include "pico/stdlib.h"
#include "pico/binary_info.h"

#include "hardware/flash.h"
#include "hardware/sync.h"

#include <lfs.h>

// Define the flash sizes
// This is setup to read a block of the flash from the end 
#define BLOCK_SIZE_BYTES (FLASH_SECTOR_SIZE)
#define HW_FLASH_STORAGE_BYTES  (1408 * 1024)
#define HW_FLASH_STORAGE_BASE   (PICO_FLASH_SIZE_BYTES - HW_FLASH_STORAGE_BYTES) // 655360

int pico_read(const struct lfs_config *c, lfs_block_t block, lfs_off_t off, void *buffer, lfs_size_t size)
{
    uint32_t fs_start = XIP_BASE + HW_FLASH_STORAGE_BASE;
    uint32_t addr = fs_start + (block * c->block_size) + off;
    
    // printf("[FS] READ: %p, %d\n", addr, size);
    
    memcpy(buffer, (unsigned char *)addr, size);
    return 0;
}

int pico_prog(const struct lfs_config *c, lfs_block_t block, lfs_off_t off, const void *buffer, lfs_size_t size)
{
    uint32_t fs_start = HW_FLASH_STORAGE_BASE;
    uint32_t addr = fs_start + (block * c->block_size) + off;
    
    // printf("[FS] WRITE: %p, %d\n", addr, size);
        
    uint32_t ints = save_and_disable_interrupts();
    flash_range_program(addr, (const uint8_t *)buffer, size);
    restore_interrupts(ints);
    
    return 0;
}

int pico_erase(const struct lfs_config *c, lfs_block_t block)
{           
    uint32_t fs_start = HW_FLASH_STORAGE_BASE;
    uint32_t offset = fs_start + (block * c->block_size);
    
    // printf("[FS] ERASE: %p, %d\n", offset, block);
        
    uint32_t ints = save_and_disable_interrupts();   
    flash_range_erase(offset, c->block_size);  
    restore_interrupts(ints);
    
    return 0;
}

int pico_sync(const struct lfs_config *c)
{
    return 0;
}

// configuration of the filesystem is provided by this struct
const struct lfs_config PICO_FLASH_CFG = {
    // block device operations
    .read  = &pico_read,
    .prog  = &pico_prog,
    .erase = &pico_erase,
    .sync  = &pico_sync,

    // block device configuration
    .read_size = FLASH_PAGE_SIZE, // 256
    .prog_size = FLASH_PAGE_SIZE, // 256
    
    .block_size = BLOCK_SIZE_BYTES, // 4096
    .block_count = HW_FLASH_STORAGE_BYTES / BLOCK_SIZE_BYTES, // 352
    .block_cycles = 16, // ?
    
    .cache_size = FLASH_PAGE_SIZE, // 256
    .lookahead_size = FLASH_PAGE_SIZE,   // 256    
};