I would definitely visit Sintra. The Pena Palace is so unique, and there are many other interesting palaces. Don't let the crowds scare you away. We were there in April 2018. We took a bus tour, which we don't normally do, but we were in Lisbon at the end of a 3-week trip and decided to take the "easy" way out by booking a bus tour. So we didn't have to wait in line to buy tickets. There was a long line at the ticket entrance but other than that, we didn't think Sintra was overly crowded like Disney World. And Andrew is correct about the long lines waiting to get on the trams, so we didn't take any trams. Maybe that is blasphemy! But it didn't ruin our trip, and we fell in love with Lisbon.
And Connie has the right attitude! "Expect to find lots of other tourists, go with the flow and have fun." For example, we arrived at Jeronimos Monastery early in the morning, and we got in the shortest line, assuming we would get in quickly. But we were wrong because they allow bus tour groups and cruise groups in ahead of people waiting to buy tickets. We waited for an hour, but had a fun conversation with people in line. Did waiting ruin our experience at Jeronimos Monastery? Absolutely not! We loved it!
Regarding allocation of nights for your locations:
Lisbon - 3 nights in Lisbon is the absolute minimum.
Seville - 4 nights in Seville is very good; however, you might not be able to see everything you want to see if you visit Cordoba for a day from Seville. I highly recommend, if you can, spending 1 or 2 nights in Cordoba. The Mezquita is amazing, but there is more to Cordoba than just the Mezquita. If you don't have the time to overnight in Cordoba, then definitely visit it as a daytrip, but keep in mind you will be shortchanging Seville, unless you travel very fast.
Granada - 2 nights is the minimum for Granada. I recommend seeing the Alhambra at night, too, in addition to the day time visit.
Barcelona - 4 nights is the absolute minimum for Barcelona. Keep in mind that many of the popular sites now require advance purchase tickets, so this might limit how much you can visit in a day.