Which UI updates are objectionable to you? Here's a dump of the changes I make on my system when installing Steam. It's not all of them, just the most generally useful.

To keep Steam from starting when you log in, select "Steam" from the menu bar and then choose "Settings". Under "Interface", untick "Run Steam when my computer starts". While you're in here, uncheck "Notify me about additions or changes to my games, new releases, and upcoming releases" if you want Steam to not tell you about those. You can use "Set Taskbar Preferences" to select what options appear in the right click menu from the taskbar icon.

I run Steam in Small Mode, which makes it look like the old, old version of Steam before they introduced the full screen library. To do this, go to "Steam"->"Settings" and bring up the "Interface" group. Set "Select which Steam window appears when the program starts, and when you double click the Notification Tray icon" to "Library". Click okay and then select "View"->"Small Mode" to show the classic small Steam interface. To the right of the search box at the top of the screen is a selector where you can toggle what is shown - I set this to "Installed" so only installed games are shown. See here[0] for an example. This UI will revert if you uninstall a game - just go to "View"->"Small Mode" to set it again. If you set it before closing Steam then the next time you open Steam it will start in this mode.

To avoid being logged into the Friends system by default, open the "Friends" menu from the menu bar and choose "View Friends". Then click the cog in the upper right hand corner of the window that appears - this brings up the settings window. Set "Sign in to friends when Steam Client starts" to off. There are a bunch of notification settings in here - set them as you will.

To make the Steam store not suck (bandwidth), go to the "Library" tab in "Steam"->"Settings" and set "Low Bandwidth Mode", "Low Performance Mode", and "Disable Community Content".

To turn off Broadcasting, "Steam"->"Settings", "Broadcasting" group and set "Privacy setting" to "Broadcasting Disabled".

You can tell Steam to create desktop shortcuts or Start menu shortcuts if you want to not have to open Steam manually to play a game. When installing a game, tick the "Create desktop shortcut" or "Create start menu shortcut" when installing and then you can start the game (and Steam with it) by double clicking that icon. There is no option to automatically quit Steam after a game exits (because of course there isn't - "wHy WoUlD aNyBoDy WaNt To ClOsE sTeAm" the fanboys go). For installed games, you can right click and choose "Create Desktop Shortcut" to get a desktop shortcut for your game. You can use the "steam://" URL generated anywhere Windows accepts URLs to launch steam games.

Some people like auto-updates. Some people hate that they suck bandwidth, especially for single player games. Unfortunately, there is no option in not auto-update the game - you can chose "Always keep this game up to date", "Only update this game when I launch it", or "High Priority - Always auto-update this game before others" in the game properties. Setting this to "Only update this game when I launch it" will try to perform updates when you start a game - if you have this set and you have the downloads set to not download while in game, you should be able to force launch a game without updates and download will not start.

Set up "Offline Mode" now. If Steam is down and you haven't used "Offline Mode" recently then you won't be able to play your games with Steam offline. Choose "Steam"->"Go Offline..." to run Steam in offline mode. This has the benefit of being disconnected from the Steam service, updates, friends, etc.

[0] https://imgur.com/WOUpLkr