Considering the cost of the vane gear, even the Jean-du-Sud, ($3460 CAN) plus the added bits and pieces you'll need (roughly $500), plus the cost of installation (unless you are an accomplished craftsman yourself), you'll be looking at roughly $5,000 USD, or more, for an installation on a boat that you don't intend to keep that will add only a modest value when you do sell; and, which you only intend to own for another year or two, in an environment where you can only realistically sail in comfort for 6-7 months a year. That sounds like an awfully expensive self indulgence compared with the alternative of simply increasing your battery and charging capacity and getting your autopilot properly dialed in as others have suggested, above. We sailed a 29 foot boat with an ST4000 for 15+ years, powered by a couple of deep cycle Group 27 batteries when not under power, and managed to cover most of the west coast from San Francisco to Mexico without difficulty. There is no reason why you cannot/could not do the same with your existing arrangement as noted above.
But, of course, if you have money to burn, and/or are merely looking for exogenous validation of a choice you have already all but made, buy all means, go for it. Or not.
It's worth my examining, to be sure! (And if I'm 'merely looking for exogenous validation' I'd better examine THAT even more closely. As in, kick myself in the nuts over it.)
When I'm bashing upwind on the Chesapeake Bay - as I was doing, a fair amount, this summer - "Otto" pushes the wheel one way when the boat rolls to port, and pushes it back the other way when the boat rolls to starboard. Bzzzt ... bzzzt. Bzzzt ... bzzzt. It appeared to me that the flux-gate compass, which is mounted near the waterline, close to the mast, and (as far as I could determine) away from any magnetic influences, was dictating this behavior.
I set the response level to "1" and I got the same behavior. Was it because my sails were unbalanced? I adjusted them as closely as I could to a neutral helm, but it didn't seem to help. That response level was the only thing I could adjust on the new Raymarine autopilot.
Maybe more solar panels would keep up with this, and they would be less expensive (though no less permanent) than a Cape Horn wind-vane. Maybe I could add a bigger battery-bank, too ...