I'd like to be able to have a column on estimates and invoices to set a discount for an item rather than a discount from the whole price. Is this possible? Many thanks

Per line item discount rather than whole invoice discount is currently available in only the QBO products for the country of India. No clue when it will be released to the rest of the world but when it is you will find an option in company settings - sales - sales forms content to specifically turn on line item discounts