any suggestions for similar - slow and thoughtful - spy movies?

The Falcon and the Snowman. How did no one bring this one up? Savages.

Most of these have already been recommended, but these can get your started

The Black Book (Dutch)
Flame and Citron (Danish)
The American (Clooney)
Army of Shadows (French)
Good Shepherd (Matt Damen)
Soldier of Oranje (Dutch, with Rutger Hauer)
Max Manus (Norwegian)
Syriana (Clooney, has some explosions)

And not exactly slow and thoughtful, but after seeing it when it came out, my first thought was "Clooney needs to make a full blown spy movie", I give you....
Confessions of a Dangerous Mind

"Loves turned to lust and bloods turned to dust in my heart"

I'd recommend Spy who Came in from the Cold, which to me is the gold standard of these kind of espionage movies. Also, for a more comic take, Our Man in Havana. Also, the cable TV series The Americans, and the British production Cambridge Spies.

"The Farewell Affair" or "L'affair Farewell" by Cristian Carion. Less known, but well worth to check out. Very Le carré cold-war stuff with a nice (but tiny, yet perfectly cast) role by Fred ward as Ronald Reagan.

And both with a bit more action, but still worth looking up within this category:
"The International" by Tom Tykwer.
"Traitor" by Jeffrey Nachmanoff with Don Cheadle and Guy Pearce.

From the comments it seems you've read and seen most of Le Carre.
I'd suggest Len Deighton if you want to continue in the genre. A bit dated sice the wall came down, but still very good imho. Film/TV adaptations are rare but available if you look hard enough. :/

Also try Martin Cruz Smith. Start with the movie "Gorky Park". :)

The Good Shepherd

Three Days of the Condor
Spy Who Came in From the Cold

Not necessarily a spy movie, but you might enjoy The Osterman Weekend

If you want a funny one, try the OSS series with Jean Dujardin. Fun 007 spoof

Three days of the condor, The Ipcress File, The Odessa File, The Life of Others, The Conversation, The Breach, Syriana.