We were in Portugal early spring of 2023, a lot of Lisbon (and Porto) was extremely crowded. If we would have skipped every place that was crowded or had a line, we wouldn't have seen much.. The challenge is to figure out a way to manage the crowds and do what is important to you. It doesn't sound like Sintra is that important to you. When are you going? If you are going at a busy time which is probably April thru October, maybe skip Sintra and visit if you come back during off season.
For me, a brief visit of a bunch of the palaces doesn't allow one to really experience Sintra anyway. I thought the Regaleira grounds were beautiful and fun. We noticed that on the day we were there, after 2pm, things really calmed down. The initiation well line was at it shortest after 3. I believe this is because the tour groups thin out as the day goes on. I didn't think the Regaleira palace was at all interesting. There was wait to get in and we shuffled through it with the masses not able to see much. I thought the Regaleria palace was more crowded than Pena palace interior and the flow through Regaleria made it more difficult. There is a central hall and you stop and peak into each room. The hall was incredibly crowded and as a 5'-3" person is was hard to get a peak. Now we did the Regaleria palace at about 2:30, and maybe the crowds are thinner at the end of the day. Some folks said for Regaleria, go to the well, first thing in the morning. Others said to go to the palace first. Others said that the tour groups arrive in mass at 9am. Its probably all true, including the experience we had, it just depends on the day.
In contrast to Regaleria, we felt the best time to go to Pena was right away in the morning. I know you are not interested in the interior, but advice from many, the interior is less busy for the first few ticket times because folks are not given a visit length. As the day advances, there are more people in the palace and more people hanging out on the terraces. We did the gardens after the interior and the gardens did not get busy until about noon and on. We noticed on our way from the Moorish castle to Montserrate that Pena gardens, ticket lines were nuts in the afternoon. It was a lovely day so that may have been a factor. Also, it might be quieter at the end of the day.
I thought the Moorish castle was a great visit. We had views of Sintra and Pena and it was great to be out side. We walked to the Moorish castle directly from the Pena gardens.
OP's are looking for the exact time periods with shorter lines for the palaces. On some days later in the afternoon might be better. Another place with miserable lines in the hot sun is the Belem monastery. We got there at 9am. I asked a guard if a later afternoon visit might be better. His reply, was "sometimes". Then I asked for which days would later afternoon be better. He said there was no pattern. I think similar is likely happening in Sintra.
When we were in Sintra, the bus situation was a mess. Very crowded and unreliable. Walking and using the bus takes more time so our strategy was Uber/Bolt which worked in some situations and was a nightmare for others.
I think one of the problems with SIntra is people are looking for the perfect plan. Well, things are constantly changing there and every day can be different. I think posters for Sintra are getting all kinds of differing advice because conditions can vary day by day and week by week. Also, people can have differing experiences and can, do, and are allowed to have differing opinions. For some people, a wait might be worth it for places like Pena and the Jerónimos monastery. For me, I planned for both those venues carefully taking in many opinions and for us, our strategies worked and we enjoyed our visits.
ADDING: I saw the Palace of Queluz in the comments. We didn't go, we couldn't fit it in. People seem to really like it. Its easy to get to and at least for now, less busy.