You can try Steam Small Mode or Steam Developer Console. Other than those two I've no idea.

If you really don't want to use Steam Client, there is always the option to switch to GOG and purchase your games there if and when available. GOG allows you to download offline installers that you can, once downloaded, use to install and play even without internet and do not require any client to function. At least the single player side of the game. I don't play multiplayer so I don't know if new games have attached their multiplayer to Galaxy client or not.

Downside is no game stats are counted (achievements, playtime or anything else) for your account. Whether that's a downside obviously depends whether you care for those stats or not. You also can't use Steam features like Steam Workshop and need to rely other modding sites for your mods.