Data Secrets Lox is set up as a replacement for the SSC open threads, and is run on actual forum software, which means there are topic-specific threads for easier navigation. As membership increases, I expect we'll add subforums, also.
https://www.datasecretslox.com/
The old link will continue to work for a while.
r/EnterTheGungeon and r/AceCombat were two great ones. I say "were" only because they (naturally) become sort of semi-dormant because those games haven't seen major releases in a while, so discussion has petered out. r/Xcom is pretty strong!
It's hit or miss, but a lot of "niche" hobbies have really good subreddits. I recently got into watches and subreddits like r/Casio and r/Seiko are fun and supportive. Not exactly deep discussion, because there's only so much you can say about a watch, but they are fun.
There are some decent audio-related subreddits. I'm a mod at r/budgetaudiophile and we try to be as helpful as possible who are dipping their toes into the hobby, though I'm not too active over there these days.
I haven't found any solid tech-related subreddits, ever. Though, I haven't looked too hard I suppose.