ETTR is only a useful exposure strategy at base ISO for ISOless cameras like the a7. What’s the maximum ISO you should use on the a7? There are three ways that I know to find out.
The first is to minimize read noise as referred to the input of the pre-ADC amplifier. If you’re trying to do that, turning up the ISO in the a7 past 1600 or so is fruitless.
The second is to maximize the signal-to-noise ratio in the lower parts of the midrange or the brighter parts of the shadows. If that’s your goal, you may be just as happy gluing your ISO knob to 100.
The third is to look at the Unity Gain ISO, and not use ISOs more than a stop or two over. Since the 13-bit ISOug of the a7 is about 1000, 2000 or 4000 should be your top ISO.
Here’s what I do: I give myself a stop or two extra headroom if I don’t have enough light to do ETTR at base ISO. After that, I increase the ISO as necessary to maintain that headroom until I get to 1600. That way my finder, which I usually leave in normal (non-automatic gain control) mode, doesn’t get too dim to use.