Grandkids Are Going To Bankrupt Me

You could perhaps switch it up and find another, smaller, bar/coin/round to give each that has a more similar monetary value to years passed. They'll appreciate the same, relatively.

That way they're still getting the same "monetary" gift, I'm sure they'll understand. You could perhaps use it as a visual learning experience to show them how the value of their stacks has grown!

No one would fault you for scaling back or pausing the gifts.

If they do, well, then you know it was the metal and not your love they appreciated.

I know a man from the Bronx, NY, who kept putting money into his then-2-year-old son's account.
The kid turned 18, never worked (spoiled), and was able to withdraw the money.
This kid is no longer with us, overdosed on drugs from his father's saved money.

I could pull back and no one would say boo. One set of grandkids are 4 and 7, so they have no conception as to what is going on. Their parents, while very well educated are almost as clueless. Pretty sure if I asked my daughter what the current spot price of gold is she'd reply, "Huh?" When I give them the coins/bars they put them in their safe at home and they're forgotten.
Daughter #2 has a 13, 10, and 6 year old. I don't think they even know about what I've done. My daughter has a PhD, but would probably answer the spot price question like her sister. My son-in-law is somewhat knowledgeable and might get close. They also say, "thank you," and throw them (the metal, not the kids) in a large safe they have at home.

I don't think it's wise to automatically buy a liquid asset regardless of price. Investments are investments. When one asset class gets overheated, you switch to a currently undervalued class. Perhaps you would be better off contributing a cash amount to a 529 plan or trust.

Not a bad suggestion.

Switch to giving them each one 2 ounce silver coin or round. The UK makes some incredible 2 ounce coins.