I wonder if your "flat DTM" is still highly spun.
Did it really go "POW", or did it pow with a hiss?
Most players can add easily 30 by flattening out their fastest spin serves, if the flat is pure "POW" with no hissing sounds.
And isn't your second serve very similar to your first? That would mean you should be able to increase the first flat speed by quite a bit.