OK, so what you have made is not really sake as we think of it but Doburoku. What we think of and call sake outside of Japan is what is technically or officially called Seishu. For those outside of Japan this is a little confusing but in Japan sake is a more general term for alcohol. Anyway Doburoku is kind of a farmers quick and dirty version. It is normally drank with the lees so it can be quite chunky at times. This generally helps the brew to taste a little less sour. The other thing that helps to keep it from being too sour is that by adding everything at once the sugar level in the ferment climbs pretty fast and weakens the yeast so the yeast give up before it ferments all the sugars. However, in your case, adding 5 packs of yeast may be countering this somewhat.
Anyway, to answer your question, if it is too sour to drink a 6 oz. glass it is too sour. You'd really like to be able to enjoy a 6oz. glass or several glasses.
The sake at the top of your carboy can be siphoned off and should be fairly clear, that is without chunks and stuff to vary clear.
If you want it to last long enough to drink it all, or even the clearest half you should refrigerate it as soon as it is done fermenting as possible. This will slow the growth of souring bacteria. You should also consider pasteurizing it.