Upvote:2
The staff at Zoom SkyBar, Anantara Sathorn, is quite cool. You might get away with a tripod for a short while if you communicate with them ahead of time. The view is very nice. 360Β° view of Bangkok.
Another place with a great view is the Scarlett bar restaurant at Pullman G, Silom. If you're a guest at the hotel, you can pretty much get the run of the place if you make friends with the staff -- especially the [farang] managing staff (they love to have good reviews on TripAdvisor, wink wink hint hint).
Another hotel with a slightly more restricted view is the Novotel Ploenchit, on Sukhumvit. The executive lounge is on the top floor (29th) has an outdoor area that is seldom used. Again, if you're a guest there, you get pretty much the run of the place, and the ladies running the lounge are super friendly.
Upvote:3
Sirocco atop the Lebua Hotel and Vertigo/Moon Bar atop the Banyan Tree should check all your boxes except the tripod ban. They're bars, so reservations are not required and you can get away with just one (expensive!) drink, although depending on time/day cover charges may apply. A minimal dress code applies (no sandals, no shorts), but you don't need to rent a tuxedo. And they're both near the river and fully open air, with no glass walls etc.
The tripod rule, though, is going to be difficult: this is usually assumed to mean commercial photography. You can ask ahead of time, or just bring along of those small, cheap portable ones and plead ignorance in the unlikely event of anybody raising a fuss.