Nickel has many well proven uses by organisms, and it certainly may be worth maintaining. It is a critical element for the functioning of many enzymes.
You recommend dosing aluminum? Well, we can have the identical debate on it since it has no known biological role.
You asked:
"Should we just start tossing every trace element that hasn’t proven beneficial or is it better to mimic levels seen in seawater?"
and my answer is ABSOLUTELY ignore them. Do you realize how many elements you'd be monitoring and dosing? It would be an insane endeavor for a hobbyist.
Here's a list of what you would need to dose and monitor (or, keep from rising above natural levels, which might be harder than dosing):
Actinium,
Aluminum,
Americium,
Antimony,
Argon,
Arsenic,
Astatine,
Barium,
Berkelium,
Beryllium,
Bismuth,
Bohrium,
Boron,
Bromine,
Cadmium,
Calcium,
Californium,
Carbon,
Cerium,
Cesium,
Chlorine,
Chromium,
Cobalt,
Copernicium,
Copper,
Curium,
Darmstadtium,
Dubnium,
Dysprosium,
Einsteinium,
Erbium,
Europium,
Fermium,
Flerovium,
Fluorine,
Francium,
Gadolinium,
Gallium,
Germanium,
Gold,
Hafnium,
Hassium,
Helium,
Holmium,
Hydrogen,
Indium,
Iodine,
Iridium,
Iron,
Krypton,
Lanthanum,
Lawrencium,
Lead,
Lithium,
Livermorium,
Lutetium,
Magnesium,
Manganese,
Meitnerium,
Mendelevium,
Mercury,
Molybdenum,
Neodymium,
Neon,
Neptunium,
Nickel,
Niobium,
Nitrogen,
Nobelium,
Oganesson,
Osmium,
Oxygen,
Palladium,
Phosphorus,
Platinum,
Plutonium,
Polonium,
Potassium,
Praseodymium,
Promethium,
Protactinium,
Radium,
Radon,
Rhenium,
Rhodium,
Roentgenium,
Rubidium,
Ruthenium,
Rutherfordium,
Samarium,
Scandium,
Seaborgium,
Selenium,
Silicon, Silver,
Sodium,
Strontium,
Sulfur,
Tantalum,
Technetium,
Tellurium,
Terbium,
Thallium,
Thorium,
Thulium,
Tin,
Titanium,
Tungsten,
Uranium,
Vanadium,
Xenon,
Ytterbium,
Yttrium,
Zinc,
Zirconium