There is a work around for this issue, see the answer marked as correct in this thread:
If songs are matched by iTunes match they do not count towards your
25 000 song limit.
Depending on your storage space internal or external you could make a copy of your itunes library, and use that copy as the one you reduce/split up, keeping one copy as a back up. If you are using time machine you will already have a back up of you iTunes music anywhay, but i would make another copy just to be on the safe side.
some times you have to knock a few walls down to get that extention on your house.