Welcome, Julian. Mysteries & Thrillers is relatively inactive as forums go. The best forum to say hello and introduce yourself and your work is in Premium. Next to that, Medieval/Fantasy Magic is a diverse mix of writers, including me (writing a supernatural thriller). Others there write fantasy and sci-fi in spite of the legacy group name. Each long term member of the group usually has a long-running brainstorming thread in that group's forum, where we exchange ideas. Mine remains the most active, called The Lord of the Earth. Ignore most of the first ten pages; it was mostly me going insane trying to find an unused title for my series. I also post comments there about books I'm reading (working my way through the Dune universe) and critiquing plot holes in Gilligan's Island episodes. :-)
If you don't think you'll have a need for a brainstorming thread, you can still post your mystery/thriller stories in this group (it's one way for people to find it), but be sure to cross-post to Premium, as that's where most of the users are. For writing or site questions of virtually any type, you can try your brainstorming thread, or post it to the Premium forum. The latter will probably result in the fastest and most complete responses.
Again, welcome!
Dirk