Skyscrapercity.com for architecture, construction, urban issues etc. One of my favs, mainly because it's a old school forum so no upvoting/downvoting, no algorithms and no teenagers. Quality may be varied though - depends of which regional subforum you are browsing.
Reddit - geopolitics, philosophy, printSF. Avoid main subreddits and use multireddits feature to connect similar subreddits into one feed.
slowernews.com