Chronostasis runs on Windows, macOS and Linux. Steam copies running under Proton are fully supported.
It ships as a desktop app and as the chronostasis command-line tool; if you're not sure which you want, take the app. Every release is also on GitHub.
The modding tools come with both: the Modder tab in the app, and the format tools on the command line.
chronostasis install walks through the whole setup: it finds the game, installs the proxy DLL that carries the runtime fixes, offers the 4GB patch, and asks whether to unpack the game for mods.
Under Proton, the installer prints a launch-options line at the end. Paste it into the game's properties in Steam and you're done.
chronostasis mod install <pack> handles regular modpacks, existing Nova Chrysalia packs included, and chronostasis mod hd-install <folder> takes care of the community HD packs. The full command list lives in the CLI reference.