most efficient way to put data into data stores?

Well, there are many ways to go about this to make it the most efficient but what you could do is save each player's userid as the key, and save a table of the names of all their gear to the key. You could keep the gear in the serverstorage and sift through all the strings in the table and use findfirstchild to get the gear.

local datastore = game:GetService'DataStoreService':GetDataStore("gears")
local serverstorage = game:GetService'ServerStorage'


game:GetService'Players'.PlayerAdded:connect(function(p)
local key = p.UserId
	p.CharacterAdded:connect(function(c)
		local data = datastore:GetAsync(key) 
		if data and data ~= nil then
			for i,v in pairs(data) do
				local found = serverstorage:FindFirstChild(v:lower(),true)  
				if found then
					found:Clone().Parent = p:WaitForChild'Backpack'
				end
			end
		else
			datastore:SetAsync(key,{})
		end
	end)
end)

that's as far as i'll go to show you how to do it