Specifically for macro photography, APS-C has the advantage. The shallow depth of field in macro, becomes even more critical with full frame. In practice, a photo you are used to doing with F11 aperture with APS-C, will need F19 apertura to get the same depth of field with full frame.
In fact, that's not true. It's one of those 'you can't have your cake and eat it, too' situations...
If you match framing, you do get deeper DoF with APS-C as in your f/11 vs. f/19 example. So...you stop the lens on FF down to f/19, problem solved. For the sensors under consideration, diffraction will be no different at those apertures (you'll lose a little bit of sharpness with both). If you need the same shutter speed with ambient light, you raise the ISO to compensate – there's no noise penalty with the better high ISO performance of FF, and if you don't need to raise the shutter speed you have less noise with FF.
If you match magnification (same distance to subject), the DoF with APS-C is actually shallower
than FF at the same aperture. You have to stop down a bit with APS-C...and if you're at f/11, the FF image isn't being softened by diffraction, but the APS-C image is losing sharpness, and it loses more if you stop it down to match DoF.
So, APS-C really offers no advantage over FF in terms of DoF. However, there are potential advantages in other areas. If you match framing, you're further from the subject with APS-C. The greater working distance is an advantage when shooting skittish insects. If you match magnification, you get more pixels on target with APS-C (assuming your subject is smaller than the APS-C sensor).
The OP mentions an issue with noise in the ambient background of shots with the T1i/500D and ring flash; the 7DII might be slightly better in that regard, but a FF sensor will bring noticeable improvement.
Personally, I prefer FF for macro work (based on my experience having the 5DII and 7D concurrently). Here's a shot with the 100L, no way I'd have used this ISO with APS-C...
EOS 1D X, EF 100mm f/2.8L
Macro IS, 1/160 s, f/11, ISO 6400
Given the OP's stated uses, I honestly think the 5DIII would be the ideal choice. It's the best of both worlds (great AF, great high ISO performance, costs less than the combo of 7DII and 6D). If that's out of budget, the 7DII is probably the better choice. The AF should be great for BIF, but it's worth noting that BIF requires high shutter speeds, which often means pretty high ISO, especially with an f/5.6 lens – APS-C is fine if there's ample light, but with heavy overcast or at sunset, often not so much.