A lot of people (me included) add more apple juice concentrate at bottling to crispen the flavor. This is also high in sugar and will facilitate carbonation.
My last 1 gallon batch got a full tube of concentrate and a quarter-cup of maple syrup at bottling, and I got a solid PFSSSHT and steam upon opening after 5 days. Cider will usually pop a little after uncapping in just a day, so I go by the CO2 and vapor swirling in the neck myself.
I have never used tablets simply because it's so easy to flavor and add sugar at the same time.
A few cider recipes skip a proper secondary and just involve bottling IMMEDIATELY, if you also add sugar at this point it will carb up VERY fast.