How should I build my store for complex object with methods?

My object is like:

const inventory = {  maxSlots: 24,  content: [    {id: "item_01", quantity: 5, slotId: 0},    {id: "item_02", quantity: 2, slotId: 2},    {id: "item_03", quantity: 1, slotId: 6},  ]};

I need custom methods like:

• inventory.getRemainingSlots()
• inventory.getFirstFreeSlotid()
• inventory.addItem() (this one depends on inventory.getFirstFreeSlotid())

How should I make this object reactive?

• Single writable?
• Custom store with get, set, update and subscribe methods?
• Class instance with writable?
• One writable for each single property, then a store of stores?

I've reached this point so far:

inventoryStore.js

function createStore() {    const { subscribe, set, update } = writable({        maxSlots: 8,        content: [            { id: "item_01", quantity: 5, slotId: 0 },            { id: "item_02", quantity: 2, slotId: 2 },            { id: "item_03", quantity: 1, slotId: 6 },        ],    });    return {        // Store methods        subscribe,        set,        update,        get: () => get({ subscribe }),        // Custom methods        getContent: () => {            // I can't use this in .svelte files             return inventoryStore.get().content;        },        getFirstFreeSlotId: () => {            const content = inventoryStore.get().content;            const maxSlots = inventoryStore.get().inventoryMaxSlots;            for (let _slotId = 0; _slotId < maxSlots; _slotId++) {                const foundItem = content.find(_x => _x.slotId === _slotId);                if (!foundItem?.id) {                    return _slotId;                }            }            return -1;        }    };}export const inventoryStore = createStore();