I am sure that we have a WOB that all spren are pieces of Honor and cultivation. However, they existed before Honor was shattered. But even with these tiny pieces of investiture Honor was still Honor until he was shattered. So, how much of Honor does Dalinar need to become a shard. Also, I assumed he ascended and owned at least one piece of Honor ( stormfather) if not more at the end of Oathbringer. But sometimes assuming is bad in these books.