Hindsight is what can cause you to kick yourself...yes, it is likely the boiler is oversized. An outside reset control would adjust the boiler setpoint to the need, rather than using a fixed temp. Multiple zones may be making the problems worse with a big boiler since it can't modulate down far enough to accommodate the actual load except maybe on the coldest days. The baseboard heaters have a variable heat output that isn't quite linear, especially as you get approach the lower range, but they still produce heat. The idea of the cooler radiators is to adjust it to your need and to get the return water cool enough to allow the boiler to run in condensing mode, reaching its maximum efficiency.