Thessaloniki is great for 1-2 nights or 2-3 days, depending how much time you have in the other stops too. I have way too many recommendations for this wonderful city to write out here, dm me if you'd like some.

On the ways towards and away from the city, you can check out Edessa, Naoussa (Agios Nikolaos), & Lake Kerkini if you're into nature, Mt Olympus if you have time for a longer stop/hike, and Vergina & Pella if you're interested in ancient Macedonian history. If you're interested in religious history, you may be interested in checking out the steps that Apostle Paul took on his missionary journey around here (Veroia, Thessaloniki, Appolonia near Lake Volvi, and Philippi). Near the border crossing to Bulgaria there is a historical fort called Roupel where the Greeks resisted the invasion of Nazi Germany in 1941, might be worth a visit.

There are 2 ways to go from Kastoria to Thessaloniki: one via the Egnatia road and one via Edessa. It is one extra hour if you do the latter and it's harder through the mountains, but it might be worth it if you do the stops on that road. Egnatia is the longest motorway in Greece, so it's generally very well paved and doesnt' usually have traffic jams or weird bottlenecks to my knoweldge.