I also use an acetate bath to nickel plate. To produce enough plating passivation sufficient to stop iron content parts from rusting only needs fractions of microns. The plating thickness is too small (for me) to measure. Usually, about 10 minutes to 15 minutes at a current between 0.5A and 1.0A is enough.
If the part is to be used, and faces some wear, then leave it in for about 45 minutes.
I have found that as the solution is used, it gets darker green, up to a point where it goes stable. Several YouTube contributors recommend some salt in the solution to increase conductivity, and this works, I have found that if the solution is left for some weeks between times of usage, it goes a dark brown colour, but still plates nickel. This is not a problem, because the nickel has to be in there, and the anode can only pass more nickel into the solution. I simply added some diluted battery acid (H2SO4) a little at a time, while stirring, and the solution went back to bright green.
My starting point was about 3 or 4 years ago, from Geoffrey Croker's very basic video on how to easily get plating. I have had some strange results, including one where the (beautiful) plating came out contaminated slightly to make a slightly gold (ish) tinted colour.
I did learn that one has to use the pure spirit vinegar. Various "pickling" vinegar products look OK, but contain sugars, and other flavourings. You have to read the label carefully. Do not use brown vinegar. It maked a horrid dark sticky mess. Clean spirit vinegar works every time
I am not sure what happens if there is iron in the anode, but it does not seem to end up on the part being plated.
12 volts, from a supply capable of many amps is not a good idea. Certainly, do not use a battery. Any small ex-phone charger, or similar little supply can do. They may be only rated 5V, or 3.3V, or 4.7V, whatever, but will current limit at less than 1A. It kind of overloads the little charger, and the voltage will end up at something quite low. I used a current controlled bench supply, and I discovered that a good conductive solution, running between 600mA and 1A, usually has about 3V to 4V across the electrodes. None of this is very critical. It just works easily.
Thanks for posting your plating work. You were plating onto zinc, which was itself previously plated onto the bolt.