Hi Dominik,
Without customizations the only way I can see this is with some manual intervention to split up the invoices such that all items on each invoice share the same payment terms. From the complexity of the request I'm guessing you already know all about payment terms and early payment discounts.
I would see if the client is willing to simply generate multiple invoices for these types of orders. This could be a pain, certainly, but I'm guessing their clients would also make a ton of payment errors if the payment terms were variable per invoice line.
One helpful automating step would be to customize the sale order lines so that they can be linked to payment terms. This would allow overriding the invoice creation step to generate separate invoices where lines are grouped by common payment terms. I can pretty much guarantee that such a module would break a ton of tests, however.