I’d recommend looking at some other magical items (I’m assuming this is a magical item at least). They might provide some good insight in terms of helping you understand designing and formatting.

But as a base, I think 1d6 or 1d8 would be good for damage. It’s less than a high-level Monk’s unarmed strikes, but it roughly balances out in scaling due to the Proficiency Bonus being added to its damage. You’ll also want to specify that the bonuses (the +PB damage and the extra Ki Point) are only granted while the monk is attuned to the item.

The bayonet pistol is a little more tricky. I think it makes sense that the gun could be unfolded from the palm as an action, and I’d recommend having the bayonet also unfold as part of that action or else it’ll take up too much of the monk’s action economy. When the bayonet pistol is out, have the monk’s melee attacks deal an additional 1d4 piercing or slashing damage (pretty much adding a dagger’s damage to the base). But regarding the gun, it’s worth noting that firearms aren’t considered monk weapons so most monks aren’t going to be proficient with them.