I've heard the mantra of making a GTC order at a high price to prevent the broker from lending out the shares. Different brokers allow different maximum prices for a GTC order: Fidelity's maximum is lower than many others, so often I don't do that as the price is too low for me.

I've also heard that the brokers being able to lend shares is a major source of liquidity, altho I suspect much of that liquidity is mostly of benefit to shorts.

Perhaps the thing to do is put them all in a cash account. Certificate form of course guarantees no one can borrow them, but it might take a week or more if YOU want to sell.