I’ve been using a 5 devices pack for years, but now I only need it for one PC and I need to renew it soon. Is there a way to get a (cheaper) renewal for one PC only? If not, what should I do?

While there’s no cheaper renewal per se, you can simply buy a subscription that is for up to one device, which is a cheaper option than a license for up to five devices.
Practically, you would not renew your Norton 360 Deluxe, but would buy Norton 360 Standard instead.

Keep in mind that the automatic renewal for your Norton 360 Deluxe should be turned OFF. Here is the instruction for how to manage renewals in your Norton Account.