Upload & Sell: On
I use the Zeiss 18/3.5 ZE, TS-E 17/4L, and TS-E 24/3.5L II. I owned the Zeiss 21/2.8 ZE for a few years, but sold it after getting the TS-E 24/3.5L II, because (as you suspect) it's too close to the 24mm TS lens, and not as good. After getting the TS-E 17/4L, I realized that I didn't have a relatively small ultra-wide (compared to the TS-E lenses, at least), and so I bought the Zeiss 18/3.5 ZE. It's an excellent lens.
I posted some threads to compare the Zeiss 18/3.5 ZE with a few of the usual suspects. I think the best one is,
I'm in the middle of a series comparing the 18 ZE, Oly 18/3.5, TS-E 17, and 16-35/2.8L II, but the weather isn't cooperating. So far, all are excellent, and so you can't go too far wrong.
I've attached a few more comparison images below, that I don't think I've posted before. They're 18 ZE (top left) and Ef 16-35/2.8 L II on 1DX, tripod, LiveView MF, etc., taken about one year ago. The first three are at f/3.5, and the last at f/8.