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_{At f/5.0 the 28-300 is not as sharp as the 70-300 at f/4 and the 70-200 blows both away at f/2.8!}

_{At 70mm and f/5.0 the 28-300 is not as sharp as the 70-300 at f/4 and the 70-200 blows both away at f/2.8!}

I'd like something that performs excellent from around 100mm to 200mm, and can take crappy photos from 30mm to 100mm if I need to).

I was originally looking at the Tamron 18-400mm, but the images don't seem to be very sharp (as would be expected of a lens with that much range).

The Tamron 18-400mm f/3.5-6.3 Di II VC is an APS-C only lens. It wouldn't work on your full frame 6D.

The Tamron AF 28-300mm f/3.5-6.3 XR Di VC is a FF lens, but it can't even keep up with the lowly AF 70-300mm f/4-5.6 at their common focal lengths, much less the Tamron SP 70-200mm f/2.8 Di VC (which is the older version of the current SP 70-200mm f/2.8 Di VC G2).

_{At f/5.0 the 28-300 is not as sharp as the 70-300 at f/4 and the 70-200 blows both away at f/2.8!}

_{At 200mm it's even worse at f/5.6 than the 70-300mm at 180mm and f/4.5 and the 70-200mm again is much better even wide open at f/2.8!}

In Tamron nomenclature, SP lenses are considered their premium line. Di II lenses are APS-C only, Di lenses are FF, and VC is Vibration compensation. You might be able to find a new copy of the SP 70-200mm f/2.8 Di VC just within your stated budget. Be aware there is also an even older Tamron 70-200mm f/2.8 Di Macro lens model that is not as good as the SP 70-200mm f/2.8 Di VC.

I'd like something that performs excellent from around 100mm to 200mm, and can take crappy photos from 30mm to 100mm if I need to).

The entire point of an interchangeable lens system camera is to allow you to use different lenses that are better or even great at one thing but unsuitable for other things. Fixed lens cameras force you to use a single lens that is mediocre or worse at a lot of things but better at nothing. Insisting on using a single lens for everything on an interchangeable lens camera is not much different than using a fixed lens camera. In some cases the fixed lens camera may meet your needs better than an ILC with only one lens.

The entire point of an interchangeable lens system camera is to allow you to use different lenses that are better or even great at one thing but unsuitable for other things. Fixed lens cameras force you to use a single lens that is mediocre or worse at a lot of things but better at nothing. Insisting on using a single lens for everything on an interchangeable lens camera is not much different than using a fixed lens camera. In some cases the fixed lens camera may meet your needs better than an ILC with only one lens.