I have to wonder about the smaller pulley wheel. Rotational speed is often important, but if you've already been using it for 3 years, I suppose it's fine.
As for the dimming for the window regulators, and headlights - they're very dependent on the overall system voltage. The regulator motors are NOT weak - they pull a pretty good amount of power from the system, which will pull your system voltage down from the 14 volt range into the 12-13 volt range. 5-10% is enough of a power loss to notice your lights visibly dimming, while not actually affecting your car's electrical system.
I personally have never heard of the voltage regulator being built into the ECM system. It's usually right in (or on) the alternator itself. It kind of has to be right next to the (removed) to properly control it. Remember, these aren't generators, they're alternators. (ECM, of course, then often controls the regulator module)
https://www.carparts.com/classroom/charging.htm
A larger battery doesn't really help much of anything - the battery voltage maxes out, normally, at around 12.9 to 13 volts. Adding battery capacity adds amperage without adding voltage. Basically, you get more capacity, but no more pressure. (Using a water analogy)