Looking for a great tour guide for Sintra, that would go in with us to the Palacio de Pena and explain,Also the other highlights of Sintra.
While I cannot help with contacts I wanted to explain something that may help.
There would be logistical issues with what you are requesting, not making it impossible but making it unusual.
The Pena Palace building is quite small and rather underwhelming inside considering the lavish exterior. It is also almost always packed in like sardines so a tour guide would have trouble guiding others and certain the palace is more interested in the numbers than helping a tour guide have more space. Nor do I feel like it was a place inside where a guide would be helpful to have. Signage is good and it is as I mentioned small and very fast moving.
The bigger issue though is understanding the logistics of reaching it.
It stands up on a large hill ; to get there from town one needs to take a bus, taxi or private car. Private car issue is there is not close by parking and at times no parking so private car means lots of walking.
There may be someone you could hire but considering your 30 minute inside tour may mean paying them for a full day I cannot see any value. The guide would mostly be paid to walk and wait on lines.
The best part of the castle visit is just standing, walking or sitting in the exterior portions/balconies of it.
Not something a guide would help with.
Minus the crowds that would be really enjoyable, with the typical crowds it is only somewhat enjoyable.
The park surrounding the castle, which is really nice is a place a guided tour would actually be quite good for as it is really large, signage is not that good and there are no crowds.... I did not see any tours like that and still have logistically issues reaching it plus the park is a ton of walking hills so need to make sure the group is up for that.