The above recommendation for the Sigma 30mm Art lens is great. After I switched to a full frame my old rebel sat unused for a long time, until I bought that Sigma 30mm and decided to keep the rebel around as a camera that I wouldn't worry about breaking or losing. Totally revitalized my appreciation for the crop sensor. I often think it would be really nice to ditch all my gear and just get a solid crop sensor with Sigma's 18-35mm F1.8 and their 50-100mm F1.8... But then I am one of those who does like the (arguably subtle) difference I see with a full frame-- for me it's less about low light performance or where I have to stand in relation to my subject and more about the amount of background context I can get without losing compression or adding distortion (although distortion is an easy fix in post). The "ff difference" is completely un-noticable to someone who doesn't take photos unloosed they're coached to see the difference, and if you do take photos it only matters as much to you as you want it to. Totally a personal preference. In any case, it's up to you if it's worth it, and anyone else's advice on it is obviously based on their experiences (which almost certainly will differ from yours). I think a lot of times people think that the FF upgrade will instantly mean better photos (I was one of them and was let down when my photos still sucked), but it's just a different tool. Like buying a bigger hammer but still working off of the old plans expecting a different outcome.
You mentioned the 6D above, and I think that's an amazing bargain these days. I've been using 5diii's for a few years and recently borrowed a 6D (original, not mark ii) to find that I have had more bells and whistles than I need. The 6D takes phenomenal photos and can be found for cheap since the mark ii was released. In the next few months I'm actually downgrading two of my 5Diii's for four 6D's (need more angles for some video projects, but I don't need 61 AF points).
Someone else mentioned flash, I also think that could be a great option to get what you're looking for. If you're throwing enough light off of a Fong diffuser to get your ISO down to 100 and aperture nice and tight you'll be amazed at how well your camera will perform. Of course learning to use flash is also a great and fun new challenge. Sorry, I don't mean to assume that you don't already use and understand flash, I'm just trying to hit all of my thoughts on the subject (for anything they may be worth). Look into a Yongnuo flash and a Fong diffuser if you're not using flash already, it's a great way to start without the intimidating price tag.