I'm not an expert, but have recently found my own boat's reason for excessive zinc use.
BAD GROUND CONNECTIONS.
Take every one of them apart, make SURE they are not corroded in any way and are in fact, seriously a good connection. Make sure ALL your negative connections have a CONTINUOUS good path to a saltwater ground.
I used to go through a decent sized zinc about 3 times a year.
I finally fixed all my negative return wiring
which was factory.
1/4" spade lug terminals which were in various states of decay.
I replaced ALL of them with crimped and soldered ring terminals on ss screw terminals.
Now the same sized zinc after 3 months looks almost new, due to cutting off positive voltage leakage to ground.
All I can say is,
"Why did I wait so long to finally do this relatively easy job?"