How to safely limit current for DC12v 30a?
You don't want to spec a power supply for your project based solely upon the maximum current it can provide, as standard supplies aren't intended to operate at maximum current.  While most switching power supplies should have over-current protection to save them from damage, an over-current event would place the power supply in failure mode.  A current limited power supply is what you need.  A current limited bench/rack power supply would also suffice in your situation, and may also be an advantage based upon the fact that you could also adjust the output voltage in order to experiment with the best voltage/current combination for your application.

You mention that very low voltage is required for initial immersion, so it may be likely that a similar low voltage may be optimal for completion of the process, with 12 Volts being way too high.  If you are experimenting, then some sort of adjustable voltage/current power supply would be a necessity.  Unfortunately, bench supplies that can provide 30 Amps of current are not inexpensive.

The resistor idea is not really a reasonable solution. Even though it could protect your load from over-current, it could still shut down a standard power supply, and while in operation, would dissipate any excess power as a large amount of heat.

As a side note:  I'm not an expert on stick welders, but I understand they have very large transformers that provide very low voltage at extremely high current, with the current or voltage being adjustable.  You might explore using a low powered stick welder as a source of your power.

If you can eventually find the "sweet spot" for your current/voltage requirements, a permanent solution of a very powerful transformer may be your answer.  Many people rewind secondary coils on MOT (microwave oven transformer) cores to provide very high current at low voltages, and multiple windings (or a tap on a single winding) could be wound that could be switched between if you needed to change voltages when going from the drop stage to the plating stage of your process.  You could then use resistors to regulate your current if you already know your demands, assuming that the secondary transformer windings can provide enough current.

Alternately, you could build a transistor circuit for the MOT to fine-tune the output current limit, which would require you asking a separate question, if it has not already been asked and answered.