Some others here may have opinions on Sago pondweed control but I've found a cutter rake to be sufficient in most cases. It does take some work but this type of pond weed is not the devil of all pond weeds.
I work on a guys 2 acre pond that we manage this with rakes every summer and have even tried an 1/8" braided cable with fairly good success. We did apply chemical several years ago but I'll have to look back and see what it was we used-it worked a bit TOO well, but didn't disturb any other aquatic life (at least fish).

www.aquaticspecialties.info
May God be with the Overton's Family.