Ideas for before and after RS Basque country tour

I took that tour (2019)...it's wonderful and I suspect you will enjoy it.
Our adventure began in Paris for a few days prior to the beginnng of the Basque Tour. We took the train from Montparnasse Station to Bayone, where the tour begins. After the tour, we flew from Bilbao to Lisbon and rented an apartment near Rossio Square. After that, we took a train to Porto for five days. We were gone a little over three weeks. I should mention, we were not the only travelers from our Basque Tour to fly to Lisbon from Bilbao. Four of them had booked the RS Portugal tour immediately after their Basque Tour was over.

Are you trying to stay within Spain? Or is basically anything open?
It seems you have exhausted the parts of Spain that are easy without a car (not that other areas cannot be done that way, especially if you have three full weeks, it is just not simple), but I would struggle to create an easy itinerary after Valencia and Zaragoza to fill three weeks. I have Tarragona on my list also but was going to rent a car for the areas nearby. If you did not explore the areas north of Barcelona, those are possible by public transport.
Portugal is a nice idea for your time frame.

You could head to Madrid, but immediately take a train to Extremadura. Cities on the train line include Plasencia, Cáceres, Mérida, and Zafra. From Cáceres you could take a day trip by bus to Guadalupe for its monastery, or to Trujillo.

Another option would be the Burgos, León, Zamora, Salamanca area.

There’s nothing wrong with Zaragoza or Valencia, but I would want to spend two weeks in just those cities.

When would this trip take place?

Have you booked your flights yet, if so are you flying round trip, or multi city, and which cities?

Do you want to explore other regions of Spain or travel to another country?