So then I should only add water?
Ultimately when you choose to top up with water you are diluting your final product. Many go this route but you have other options, such as:
-craft your batch with an extra 3 cups or so per gallon and you can use a simple 750ml bottle, or glass jar, to hold any excess until needed
-top up with a similar or coordinating flavor of juice or sugar-water which is the same gravity as the starting gravity
-make a few gallons of potato wine and use it as a neutral wine specifically for topping up. If you typically make 12% batches, then make a 12% ACV potato wine
And prevent racking loss by pouring that tail end of lees + juice into best fitting jar, seal it, and refrigerate so it quickly clears and you can siphon that residual amount of must off of those lees. Hope this helps.
Far too early to tackle degassing on the first racking. You will want to let this must finish fermentation, rack again & then serially rack about every two months; maintain SO2 levels by adding k-meta on quarterly basis. Continue on this pattern until the must is clear & no longer drops sediment for 60 days. If you allow this to occur naturally without use of finings you 'should' have a clear, lees free, degassed wine in about 6 months. Just remember if you plan to manually degas it helps if the batch is at 75F, otherwise that dissolved CO2 just likes to stay in solution. Should definitely be degassed on its own at the year mark, which will be a good time to consider how to finish this and prepare for bottling, such as stabilize with k-meta + sorbate and sweeten up, or if leaving dry just stabilize with k-meta and then proceed to bottle. Classic pattern for an apple wine crafted in the range you have.