Delete (some of) them? Do you need a permanent, long record of every text "conversation"?
If for some reason you do, open iMessages on a Mac, select one of these chats, File (menu), Print, and either print to paper or PDF file. Then delete that chat and you'll have a permanent record. If PDF, Time Machine- assuming you are using it- will back it up too, so you'll have a permanent record and a backup.
I prune my own text conversations all the time. Sometimes, I'll delete one after sending a response knowing when they reply, they'll basically create a new one. This keeps Messages iCloud allocations very small.
Another option: pay Apple an ongoing subscription for more iCloud space.