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Well ... if you're striving for the foreshortening effect through to infinity ... what you really want is going to be the 17L TS-E or 24L TS-E II. If your budget is able to handle the ZE, it can handle the TS-E's ... get the right tool for the job. You won't be happy with anything else until you do.
One additional beauty of the TS-E usage is that you can achieve your immense DOF at f-stops that are not diffraction inducing, i.e. 5.6 or 8 vs. 11 or 16.
That being said, I'm still a fan of the Oly 21/3.5. ALL UWA optics have their limits and compromises ... you just gotta "pick your poison" and stay away from that which bothers you most ... mustache distortion, first order distortion, CA, vignetting, contrast, bokeh, corner vs. center resolution, size, weight, cost, etc.
The reality is that no UWA glass serves all, as certain things are optically diametrically opposed in design. Those great corners come at the expense of central sharpness or mustache distortion or are achieved by a larger image circle. Superior central sharpness comes at the expense of weak corners or 'nervous bokeh'. Great contrast comes @ the expense of vignetting ... and so the story continues.
Pick your poison ... but for your goals of foreshortening to infinity without diffraction... TS-E is the way to go. The poison of TS-E being slightly less contrasty than the ZE (very easy to dial in to taste in post) with it's larger image circle. I'll take the contrast reduction over the mustache distortion any day to achieve the diffraction reduction and DOF increase that you seek, along with the added benefit of miniscule vignetting (larger image circle). People, ooh & ahh over the ZE (and for good reason), but the "extra" contrast that people love @ the ZE doesn't help you achieve your goals @ foreshortening DOF with low diffraction the way a TS-E will.
It takes a bit to get the hang of tilt if you've never used one before to get the DOF / foreshortening effect dialed in ... but you really ought to take one out for a spin (rent, borrow, etc.) just to shoot straight ... you'll find that it is great, well corrected glass ... clinically boring maybe compared to sooc from the ZE, but great glass nonetheless that gives you a wonderful file to work from with smooth transitions (opposite the more rapid transitions employed by Zeiss).