I was thinking the exact same thing; using bottles in general for
topping is just not going to be cost effective unless it's 2 buck
Chuck (which is no longer 2 bucks). I would ether rack this into
smaller carboys if I wanted to keep it the same or I would get a nice
box of Australian box wine. That way you can top without any issues.
If you start with at least 5.5 to 6 gallons and use a 5 gallon carboy
you should have enough left to never need the other topping wine.

After I rack I always settle the lees in a fridge overnight and rack
that to another container. As long as it's over 750 ml it will last
at least 6 months to a year.

Joe