Portugal would be a good one if that's how you are leaning right now.
That said, just about any country in Europe can be great for solo travelers based on your other interests you listed. Portugal would be a good one if that's how you are leaning right now. If not, I would say Italy, Croatia or Greece and if you really want to add the beach to your itinerary, then I'd do what's needed to pull the trip up into the month of September (particularly the first half) to maximize my chances at getting better beach weather and finding more local businesses that may not have shut down operations just yet.