Looking at the best way to upload ~20,000 balances to a live parachain. Assuming will need to upload these balances across multiple blocks. Are there any examples I can of this been done already?
I don't think there are any tricks here once you have started a live chain.
Generally speaking, you shouldn't mint new funds onto a chain once tokens have been allowed to be transferred. If you have already enabled token transfers, I would not suggest minting new tokens into your economy.
Assuming you already have an account with the funds you want to distribute, you can simply create and submit batches of transfers to move funds to the appropriate accounts.