it appears the DOSBOX config put in the Steam release is not compatible with newer WIndows versions / GPU drivers / higher res displays.
Hence it only worked for me in Window Mode, while any DOSBOX option was in full screen I only got a black screen.
The easiest fix without fiddling a lot but being proper and supported, is using the community patch “LCGC Systemshock Portable”, also known as “SSP”, it also offers a few enhancemens and installs using the DATA files from your original game installation (in this case Steam’s).
https://www.systemshock.org/index.php?topic=211.0&utm_source=chatgpt.com#post_downloadssp
The only thing not working is the option “Use Gulikoza’s custom DOSBox build”.
Tested and confirmed working on a 2K Display (2560 x 1140) with Windows 11 26H2 with Nvidia Game Ready Drivers 617.14)