First of all, closet modulars do not make adequate bookcases for homeschooling. While the cubby hole look is reminiscent of my own school days long since gone by, they are simply not strong enough to be stuffed with books. I just had to prop mine with a 2" lean up with a small dresser. Now that dresser is jutting out a little, just waiting for a toe; likely mine!
Still, I used what I had, but it is time to move on. Not sure how. Money for a bookcase in the size I need isn't easy to come by. There's always a more pressing need, like food and doctor bills. But, I do believe it is time to start shopping around for either an adequately enormous bookcase, or someone who can custom build an adequately enormous bookcase.
Because, newer bookcases are a nicer option than fewer books!