I stream at 2500; 3500 should be fine as well. 60 FPS looks nice, but 30 is still quite watchable, particularly if it's smooth. I stick with the software x264 encoder, as the hardware encoder on my graphics card (labeled in OBS "H264/AVC Encoder (AMD Advanced Media Framework)" because I have an AMD card) produced really blocky video when I tried it a while ago.
The thing to keep in mind is that your bandwidth (3500kbps in your case) is limited by your Internet, the streaming service, and so on, so you have to make do with that. Encoding more pixels or more frequent frames is going to spread that bandwidth thinner. If it can't meet the demand you're placing on it, it'll start producing blurry or choppy video. Also note that high motion video is tougher to encode than low-motion: my system kinda struggles with 720p30 LoL streams, but I can stream coding sessions in 1080p30 just fine.