I have written several times about the access to Sintra by car.
Since 2019, several places are not reachable by car, and only authorized vehicles can enter, and park.
Places such as: Pena National Palace, Moorish Castle, Pena Park, but also the central square of Sintra (Sintra National Palace).
All these places are not possible to enter with rental cars.
See here and here, my previous articles with more details.
Check here for the map of Sintra, showing restrictions on car traffic: http://prntscr.com/1CmIbhk5Dz-h
(this itinerary is forbidden to reach by private/rental cars, all roads in blue are not acessible)
The suggestion would be to hire an authorized car + driver/guide or take a taxi.
The best option would be taking a private tour to Sintra from Lisbon, with pickup/drop-off from your hotel.
Check here, for the Golden Triangle Tour, one of the most popular tours.
Please check here some private transfer options.
Gonzalo
Founder/Owner of The Lisbon Guide, one of the major blog references in Portugal, established in 2014 and receiving every year 250.000 visitors from all over the world, looking to provide the best experiences in Portugal. In partnership with Portugal Magik for all private tours and transfers across Portugal, Gonzalo loves a good seafood meal at Monte Mar Cascais, and all from Michelin Chef Avillez. Favorite Hotel in Lisbon/Portugal, Penha Longa Resort by Ritz Carlton.
For over 15 years, Gonzalo have been helping thousands of travelers yearly to plan a perfect trip to Portugal. Based in Lisbon/Cascais and working in this field for over 20 years, with multiple ongoing projects. Also an avid TripAdvisor user level 6 with more than 300.000 readers worldwide.
Many years working also as a Private Guide of Lisbon, Sintra, Fatima, Porto, Douro Valley, Evora, and other locations in Portugal.