Limiting current of a transformer used for resistance soldering
If you have the budget I would recommend a suitably (over-)sized Variable transformer (Variac) for the input side. These are tolerant of transformers as loads.

If stepwise adjustment is acceptable then you could use resistors to reduce the available current.  Either on the input (need to be aware of the mains voltage dangers and switching issues) or on the secondary.

If you have a number of larger 50-75W 12V automotive filament headlamp bulbs you could connect them in series and parallel combinations to provide durable limiting resistors.