Opticallimits

Unsurprisingly the worst of the gang but still decent for a 20mm lens.

https://opticallimits.com/sonyalphaff/1116-tamron20f28

FWIW, I've added a sunstars chapter and another bokeh sample to the other two reviews as well.

It's a fair lens, in the end, as you stated in the price/performance evaluation. I see that you confirm that at ƒ/4 the light spots are "almost" circular, but not as circular as the advertising promised. It's probably the single thing that I was disappointed by.
For what concerns distortion and relatead loss of sharpness, it's a lesser problem if a) you use it on a APS-C camera b) you're doing macro (which means you most likely want blurred things around the subject) and c) you use a partial correction profile. With the last point I mean that you can reduce the distortion correction in post-processing if you are not shooting architecture (c-plan with this lens for me).

(Trying to) find a bright side: the distortion is still fully a percent (and then some) less than the result posted by EPZ. That aside, the lens still looks better than the old Sigma 20/1.8 or the Canon 20/2.8, though these were old DSLR lenses of course. The new crop of twenties is a lot better, which is also unsurprising.