You do need a hydrometer. Jacking the alcohol content up too high will make the yeast poop out, or stress out. If they poop out you get something too sweet and not fully fermented. If they stress out they make un-tasty side products.
If you want to push your yeast further, you can add more sugar as the fermentation goes along. I.e., let it ferment about halfway, add more sugar and a smidge of yeast nutrient, another halfway, add some more sugar and nutrient, etc. There are threads on how to do that... just search for "barleywine" or "high gravity beer" here in the forums. Also, champagne yeast can ferment to a much higher ABV% than regular wine yeast, but they do tend to "dry" out the wine.
If you really want to get elaborate, you can use freeze concentration to make "applejack", like they made back in Colonial America. Just search for freeze concentration here on the forums. (Since they didn't have specially bred yeast, their applejack was probably equivalent ABV% to our apple wine.) Freeze concentration is not considered distilling by the U.S. ATF, but laws may vary in your area, so it's worth a look before state cops surround your house, etc.
Also, if you want to store your apfelwein long-term, you will need to top off your carboys all the way in order to prevent oxygen exchange with the air in the airspace of the carboy.