Hi yppaul - and welcome.
As long as there is no headroom (space between the bung and the surface of the wine) more than about 1/2 - 1 inch - that is to say as long as the wine is up inside the neck of the carboy - then you can safely age your wine in that carboy for as long as you want. Think of that as "bulk aging". The reason why kit manufacturers sorta kinda rush everyone to bottle at the earliest moment is to free up the carboy so that you will be encouraged to refill it with another of their kits. An empty carboy being a terrible thing to watch.
AS to your question - is there a good substitute for K-sorbate? There is but the substitute is not an equivalent. K-sorbate prevents yeast from reproducing. That means when the yeast cell reaches the end of its life there is no next generation that will continue to ferment and that means that if you add sugar to backsweeten all that added sugar will remain as a sweetener in the wine. So, one alternative approach is to add a sweetener that the yeast cannot ferment and there are non fermentable sugars - Pure Stevia for example, but I am not sure that their flavors are neutral so the addition of such non fermentable sugars - they are more complex than fructose and glucose and sucrose - may alter the taste of your wine. But you also need to be careful - Stevia is often "cut" with dextrose and dextrose is fermentable.
Another possible "substitute" is not to depend on what is for all intents and purposes contraceptives for yeast (the K-sorbate) but is for you to actively remove every last yeast cell from the wine. You can do this by filtering the wine through what is called a sterile filter. That is a filter that blocks anything larger than .45 of a micron. At that size you will have removed every remaining yeast cell. But and that but is very big for a reason... unless your wine is crystal clear such a filter will get blocked faster than you can blink with fruit and sediment and who knows what else. But if you have removed every last yeast cell from your wine then there will be no yeast cells in the wine to ferment any sugar you add or any residual sugar that still remains after the fermentation has been halted (say, by dropping the temperature to a level too low for the yeast to function - Note that they will become active again as soon as the temperature of that wine rises ).