Is there a way to apply a discount to sales order that will apply to each line item without editing the price tier fir each item?

you can create discount item and add it after each line you want to give discount to. if you have a fixed logic when you want to apply, (to certain items, total or a particular classification) then you can write a script that does it automatically.

So there is nowhere to add a discount percentage that will then be applied to each line item?

you can use the same item at header, but then it will be off the Total, not each line item, not sure if that matters

if you add a subtotal item beneath line(s), the following discount(s) will be applied to the transaction subtotal