For a little more power out..fill the coil holes with metal as well..I just hacked sawed up a bunch of flat metal ,,glued those togeather then installed in each coil,,,😜
I agree this should get you more watts, probably higher Volts, but it might also make it cog, so that startup might be limited to higher winds.
Your next issue sounds like a controller. If your alternator is too strong for your turbine, which I'd guess yours is, then you can use a controller to help it match.
In general terms when the wind speed doubles, you have 8 times as much power available at your turbine, but your turbine speed only doubles, it's output isn't linear but cubic. As your alternator speed doubles, generally it's output only doubles. You can wind your alt for higher speeds, or lower speeds (and maybe risk burning out, though there are experts here who can help you to calculate best your winding design, I'm not one).
Fancy controllers (MPPT and low voltage boost) can be used to maximize performance over the whole turbine speed range, as well, many can as overload it to slow it down in very high winds, and pull it down below it's efficient range so it doesn't overspeed. Generally fancy controllers have cost a lot of money in the past, but there's some cheap stuff coming out of China that might be OK. There's some very cheap stuff, that is probably pretty bad.