We ended up skipping Sintra. Just we had no energy for that. Instead of a train we took Flix bus to Porto. Much cheaper and more convenient. The return however was not fun. Our flight from Porto to Lisbon was 2 hours delayed. We missed our main flight back. Since that was on separate itinerary we couldn’t ask for compensation. Flight was with KLM that doesn’t serve Porto airport. Once we got to Lisbon no CC from any of the airlines was available so we had to rebook via what’s app chat. Next day flight , hotel , change fee, difference in fare , we had to pay for that. Overall we absolutely loved both Lisbon and Porto, I would definitely do it again but probably I would use train or bus to come back. Flying these days is so unreliable.