There are numerous ways of reducing power input to your iron. As others have said and you yourself have noted, this is more likely to be of value as a learning exercise than a major improvement to the iron.
A temperature controlled iron is a really really really good idea for quality soldering.
A "closed loop" temperature controlled iron regulates the power to achieve a desired temperature by varying power based on what a sensor measures.
A power controlled iron which is "open loop" (no temperature sensor) will vary widely in temperature depending on local environment -- eg air flow & drafs, contact wit metal stand etc.
However :-) :
An extremely easy way to reduce power input is to switch a diode in series with the iron circuit.
Short the diode when full power is required.
Remember that mains kills.
Power input will be about 50% with diode in circuit.
Diode voltage rating must be mains suitable (say 2 x Vac) and current raing must at least equal iron max current
A cheapish and easy method is to use a TRIAC type light dimmer.
Wattage rating should be at least equal to iron max wattage unlikely to be a problem.
Series Light Bulb(s)
If wishing to experiment with series light bulbs place a lamp socket in series with the line.
Building this into an extension cord allows you to avoid hacking the iron.
Putting two sockets in parallel allows some interesting experiments.
Sockets with bulb out and iron plugged in will be as lethal as a normal lamp socket is. A shorting switch across the socket gives full power.
The high resistance-nonlinearity of tungsten filaments with temperature will be learned about this way :-).