Gear Recommendation Requests for Wildlife Photography

Hello! I've recently been getting into bird watching, and would like to start photographing more of the birds I see. I have been getting some good shots with my camera through my office window, but if I want to do more than that my equipment currently is pretty limited.

My Camera is a Nikon D5100, and I've had it for ages- I love this camera. I have a 55-200mm lens on it that I got way back when I got this camera, but that's a very limiting range- 200mm really isn't enough to do a lot of bird photography.

I know that when I get a better lens, it will likely be large enough to warrant a better tripod than what I currently have and possibly even a gimbal mount (though, that may be a future upgrade).

So, those are the things I want recommendations for- telephoto lenses that will fit my camera, a good tripod, and gimbal mounts. I can't break the bank, I know this will be VERY hard for good telephoto lenses but I'd like to stay under $800 for the lens if at all possible- that way I can upgrade sooner rather than later, the tripod and gimbal can be future upgrades I save up for. I'd like a tripod that will last me a good long time and is quite sturdy- the one I currently have is pretty cheap, and while it's sturdy enough for now I don't think it will do a good job with a heavy lens beyond a 200mm focal length.

Thanks!