Yeah sorry, I thought that's what I'd referred to, in my own little way
So to clarify, you were saying that you'd looked at the 17-28mm but wanted more reach, and using crop/APS-C mode gives you more effective reach (1.5x extra reach). The R models are better for this as the crop mode still retains a high number of megapixels. In crop mode you get the following megapixels
A7 III - 10mp
A7R III - 18mp
A7R IV - 26mp
Now as I said, 10mp is enough for a lot of applications and so the A7 III may still be the right choice if this is the case, but if you want to crop further still you start to be limited. With the A7R IV you can do a 2x crop and still have 15mp.
It was just a suggestion so that you could get the arguably better lens in the Tamron 17-28mm and not lose out at the long end, in crop mode you'd get 42mm at the long end