Having driven from Plovdiv via Kavala to Thessaloniki, we were planning to then head north via Rila Monastery towards Sofia. Someone has suggested a side trip to Ioannina. Given the distance, I imagine it would mean an extra two days at least.
Is Ioannina worth that extra time and distance? We enjoy walking, history, food and beer, and getting a feel for local culture.

I enjoyed it, especially the shrine at Dodomi, but I don't know that I would say it was a must see. I might rather be inclined to go Thessaloniki - Kastoria - Bitola - Ohrid - Skopje - Sofia, with Rila as a day trip from Sofia, but it depends on how much time you have. (For Ohrid see: https://mytimetotravel.wordpress.com...dmiring-ohrid/ )

Ioannina us much too far away and not worth that a big detour detour.
Gems along your Plovdiv - Thessaloniki - Sofia itinerary:
historic centre of Xanthi
archeological site of Philippi
Thassos Island
Vergina
Bitola (side trip to Ohrid)
Stobi
You may then cross the Bulgaro/Macodonian border at Delcevo and go on to Blagoevgrad - Rila Monastery.

If you decide to go straight from Thessaloniki to Sofia, you could make a detour to Melnik (traditional village with nice hiking trails to monasteries)