Will probably be better than when went went in February and nearly got blown off the face of the earth with the wind and rain (especially in Sintra!)
If you have a look on our YouTube channel, we have loads of useful Lisbon & Sintra videos we made including:-
Lisbon is a fantastic city but I'm not sure about the public transport. The thing about the metro is that it only covers certain directions - i.e. out to the World Expo site/aquarium and airport, it's great, other directions, not so useful.
We got on a tram to go to Belem one day and it broke down after 5 mins and we ended up calling an Uber. As for the "historic trams", great for taking photos, a nightmare to ride on.
The geography of the place - mains sites in 4 diffenets directions, miles apart - means that Uber saved us. Just 12 euros (one-way) to get from the city centre, across the suspension bridge and to the Cristo Rei - the highlight of the visit.
Like I said, a fantastic city, though, with loads of things to do.