There's currently a sale on for the Nikon version of the Tamron 28-75mm f2.8 zoom lens (~$400 new, $250 used). This little jewel is one of the lightest, most compact full-frame f2.8 zooms compatible with the LS300, less than 4-inches long and 500g. While this zoom is soft in its full-frame corners, it's plenty sharp within the APS-C extent of the LS300's sensor, even wide-open at f2.8 (see Imaging Resouce review below). What I haven't seen reviewed, however, is this lens' exemplary zoom behavior. It is one of the very few parfocal zooms I've found for the LS300, maintaining focus throughout its zoom range (confirmed via LCD focus peaking).

You need an adapter to mount the Tamron on the LS300, and the Metabones speedbooster and plain adapter both work well, with reliably damped clickless aperture rings. After that you'll likely discover the Tamron's main weak spot - its funky little focus ring. It turns over less than a 90-degree arc, and most of its calibrated range is devoted to close-focusing less than 3-feet away. But once you do get it in focus, there's a saving grace - engage the "auto-focus" switch on the lens, and the focus motor's gears will hold the focus ring in place.

Aside from the glass lens elements and metal mount, Tamron made this thing entirely out of plastic. It's rigid, but not really tough, and needs as much careful handling as the LS300 itself. But the two of them are so well matched, even down to the focal length markings accurately matching a full-frame FOV with the Tamron mounted on a 0.7X speedbooster at 100% VSM.

https://www.imaging-resource.com/lenses/tamron/28-75mm-f2.8-xr-di-ld-aspherical-if-sp-af/review/