There are very few truly smaller full-frame options than the Samyang 35mm f/2.8 for Sony E-mount. The main limitation is optical: on full-frame mirrorless sensors, very compact short focal length designs can send light to the sensor at steep angles, which can cause color shifts, vignetting, and weak corner performance.

If you want to go smaller, the main route is experimental adaptation: some 28mm or 35mm rangefinder or enlarger lenses can be mounted with a short helicoid adapter, creating a partially collapsible setup. These can be extremely compact and manual focus is possible, but image quality may be unpredictable, especially toward the edges.

One example mentioned is the Agfa Ambion 35mm f/4, though adapting it is not straightforward.

So in practice: if you want reliable results, the Samyang 35mm f/2.8 is already close to the practical minimum. If you’re willing to experiment, adapted rangefinder or enlarger lenses may get smaller, but expect compromises.