I'm a landscape shooter and I've owned the ZE21, 24L II and 24 TS-e...here are my thoughts on it, YMMV.
The ZE21 is a phenomenal lens for landscapes. The 21mm FOV is about perfect for me - I never feel like I need wider, it is extremely sharp, great contrast, and built like a tank. I truly have a soft spot for this lens and if you go this route, you won't be disappointed. The experience of using it and out of camera IQ are both fantastic.
The 24 TS-e II frequently makes me feel like I need to stich images to get the FOV I want which is some added work in the field and in post processing. However, the tilt/shift movements are almost indinspensible once you've tried them and the IQ is very good. Shifted panos, true/straight vertical lines and tilting for critical near-to-far focus offer nearly infinite possibilites.
The 24L is the ultimate nightscape/starscape lens. It is sharper than the ZE21 at f2.8 and has less vignetting, plus you have the option of opening up to 1.4 to really pull in light and/or shorten your exposure times. Of course, I mentioned 24mm is a little tight for me and the 24L lacks the ability of doing shifted panos like the TS-e or having an acceptable native FOV like the ZE21, so this is a drawback for me. But, the 24L has the additional benefit of being very useful in day-to-day non-landscape shooting whereas the other two are more often just dedicated landscape lenses. I've found 24mm on FF @ f1.4 is more useful than you might think for family, museum and just day-to-day shots - plus it approximates a 35mm FOV on a crop camera so it could do double duty for you. Maybe you'd find you wouldnt need the Sig 35 any longer and the funds could be used elsewhere.
So, even though I absolutely love the ZE21, I've sold it for the 24 TS-e and plan to add another 24L II in the near future. Although the ZE21 does what it does VERY well and I probably like it better than the other two on an emotional level, the other two lenses do something functional that the ZE21 simply cannot (speed or movements). I'd love to have all 3, but as long as I am budget limited, I would prefer the two with more functional benefits. If I were you, I would also be deciding between these two and weigh the costs and benefits of each for your own uses.