Canon R7 autofocus seems inferior to 80D in low light

I keep hearing how the Canon R7's autofocus is a big step up from the 90D and (presumably) the 80D. I don't have a 90D, but I do have an 80D and I thought I would test the autofocus speed difference.

I am mainly interested in using the R7 for bird photography.

I found and printed a test target (https://upload.wikimedia.org/wikipedia/commons/2/2b/EIA_Resolution_Chart_1956.svg). I mounted a Sigma 150-600 C to a tripod placed about 20-30 ft from the target, and tried a focus test with both bodies.

I use Back Button Focus (BBF). The idea was to throw the lens out of focus, press the BBF button, and see how long it took for focus to be acquired. I filmed this with my phone so, if need be, I could count the frames from press to acquired focus.

The focal length was 600mm. The shutter and aperture were set manually; the ISO was on auto. The shutter speed was set to 1/60, the aperture was f/6.3 (wide open).

I did this with two light levels. At the higher light level and smaller aperture, the ISO reading was 100; at the lower it was 3200.

For both cameras, the autofocus mode was single point and one-shot (not servo). One the R7, subject tracking was none, and eye-detection was off.

Originally, I was going to measure the speed difference, but the 80D was the clear winner. At bright light levels, both cameras seemed to lock on about as fast—if the 80D was slower, it will take a frame count using the recorded video to determine. At the lower light level (typical when photographing wildlife), the R7 could not reliably acquire focus.

This seems to match my real world experience. I went on a walk around the neighborhood and wound up with a lot of high ISO shots even with a shutter speed of 1/640. The camera fought to acquire focus. I've been shooting with my 80D/Sigma combo for some time and this has never been a problem.

Eye-tracking and all the other fancy stuff is great (when it works), but if the initial focus takes longer or just never happens, the R7 is a step down.

One possibility was that I was testing with a non-Canon lens, so I mounted my Canon EF/S 18-135 and tried the test. When the ISO read >3200, the camera failed to acquire focus; below that, it succeeded. The camera was fully zoomed (at 135mm).

I tried it at 18mm. In bright light, the focus was quick. In dim light, the AF-assist beam would flash and the focus would be acquired in about 1 second.

I keep seeing glowing reviews for the R7 (including here) and the camera sounds great on paper, which is why I bought it. So far, the purchase seems like a mistake., but there are a lot of new settings and it's my first mirrorless camera—I hope there's something I'm missing.