Yeah, the Sigma 1.8 Zooms are fantastic lenses for this Camera. Just look out to get the Nikon F-Mount Version and a Adapter for C-Mount which will let you control the Apperture, since the Nikon-C-Mount Adapter that Ships with the 2.1 Does not allow for Manual Apperture Control on these Lenses. but those Adapters are Cheap and Readily available, there Was an Discussion about this in at least one other thread about this.
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As Far as Lens Reccomendations go, the Sigma 18-35mm 1.8 and the 50-100mm 1.8 are about where its at for this camera for general use (great Value for the Money and very versatile, fast, sharp), if you want to go a little more Fancy with Lenses (and / or still play around with adapting Mirrorless Glass) there are also Speedboosters you could add to the Sigma zooms, to make them even Faster in Maximum Apperture and Wider. More on that is also discussed on various other Treads on the Forum, so i wont restart this whole Discussion about Speedboosters here, if you want to know more about the Speedbooster-Thing, reply to one of the other Threads to keep them a bit on topic, so others will find Info easier in the Future.
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Of course Lenses should always meet your needs, and there are A LOT of good Lenses out there, so think about what you want to do with them, and buy what fits you best.
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But here still some more Recommendations:
A Lens i heard from a friend, who also has an MFT-Format Camera (Havent used nor tested it myself, so cant say anything for sure about it Really, but sounds nice on paper) is the laowa 7.5mm Lens. As Far as i know one of the Widest Lenses which will cover the Full Chronos 2.1 Sensor and is still kind of affordable. This Lens however is MFT Mount, which is also a Mirrorless mount, so it will need either Modifications on the Lens itself, or you need to get an Adapter Custom made (or you need to wait until someon offers them For Sale finally). If you want some Ultra-Wide Shots, get this Lens!
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An other Reccomendation would be to get something like a Superzoom lens. 18-270mm / 18-200 or something like this. But look to get a newer version, since the older ones are most of the time not that Sharp. Those Superzooms usualy are by far not as Sharp and crisp as the Sigma 1.8 Zoom Lenses, but could be Great if you need more Range. Would however only reccomend a Superzoom Like this for shots were you are outside and have lot of light and dont plan to get crazy on the Framerate, since they give you a lot darker image (zoomed in they sometimes have like Apperture 6.2 or something, which is WAY darker than the 1.8 from The Sigmas). Also Look out to not get an AF-P Version since those are Focus by Wire and also lack Manual Control just like your Sony lenses. Nikon F-Mount AF-S is the Way to go in my opinion for a superzoom..
Furthermore for Longer Tele Lenses with good Quality there are Plenty 70-200mm f/2.8 Lenses or various Brands in Nikon F-Mount available. Newer Versions usually are quite a bit sharper than the old ones, but they are generally all considered Premium Lenses at 2.8 and are mostly not too bad anyways. I personally would avoid the really old ones made for Analog Film Cameras however, because lens design and manufacturing improved by a lot the last few years, and it shows in the final image a lot. The Older are still cheaper to get, so look what fits your butget.
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An other Lens to maybe look into is an 50-500mm or an 150-500mm Lens. They have a Smaller apperture, and are Really Fat, Long and Heavy lenses (and kinda Expensive), but they offer a ton of Reach, if you got any need for that. For the 70-200mm and the xxx-500mm Lenses i would also reccomend to stay with Nikon F-Mount and the Adapter for C-Mount which allows for apperture Control. Keep in mind that you can still use any Nikon-F-Mount Lens on your Sony With an Nikon-F to Sony-E Adapter!
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The Next ones depend heavily on what you do with your camera, and might not make Sense for everyone:
A Decent Macro Lens or Macro tubes. There Are Plenty dedicated Macro Lenses Out there, which are really worth the Money if you do a lot of Macro or "Pseudo-Macro" (Stuff that is Not technically Macro, which starts at 1:1 Magnification, but still so high magnification, that you propably run into trouble trying to do it with an standard Lens without any Hacks/ Additional Equipment like Macro Tubes or diopters). There Are so many out there, i cant really reccomend a particular one, since a lot of it comes down to personal Preference in Focal Length, Requirements on Ratio of Magnification and also on the general Design and Build of the Lens itself, look around a bit, you will find plenty of options! If you got any more in depth-Questions, let me know!.
Then Primes. There are plenty Nice Prime Lenses, which could be interresting if you carry the Camera around a lot, for example for casual shooting for fun an the Like. However, the Sigma 1.8 Zooms offer pretty decent Range and are also plenty fast and Sharp, so, if you get them Zooms, i dont know if i would get primes also myself...? Primes are in some Cases Smaller and Lighter, which can make a lot of difference if you for example just do full Resolution shots on the 2.1 and only Portrait / Close-Up stuff the Whole Time. The Sigmas still are Heavy, and depending on what kind of caliber of Camera you used to carry around all day, it could get tiring pretty fast with an 50-100mm 1.8 for example..So far a Quick overview over the Lenses i think would make a ton of sense to own for certain usecases on the 2.1, depending on what you do with it. Some People come along well with a single Prime, or a Single Zoom lens, if you do a lot of different things with the Camera, maybe get a few more Lenses. Whatever works for you.
