1.5 years sounds too soon for your barrels to be neutral. I've owned 1
American Oak barrel and currently I'm using 3 - 15 gallon French Oak
barrels. To me, the extraction from French Oak is much more subtle. I
also think that Cab Sauv stands up well to the oak flavor, that is,
it's not easily overwhelmed. Since you're using a 'broken-in' barrel
and Cab Sauv it may just mean that you need to give it time. I would
give it at least a year. Last year I put my Cab Sauv into to 2 new
French Oak barrels. It is still there doing nicely and IMO is not
anywhere near being over-oaked. Topping up twice per week for a used
barrel sounds like a lot. I top up twice per month and haven't had a
problem.

When changing wines, I give the barrel a couple of good rinses of very
warm water and even let it sit a few minutes. Tartrates that deposit
on the inside of the barrels will require some heat for them to
resolubilize. I find this helps get them off the barrel walls. It's
not perfect but it helps.

RD