Uninstalling SideMini

In this order

1. If you used iPad Only mode, open Settings → iPad Only and press Reset to Default. This turns automatic login off and starts turning FileVault back on.
2. If you used Server mode, open Settings → Always On and press Restore Previous Energy Settings.
3. Turn off Start at login so no launch agent is left behind.
4. Quit SideMini from the menu bar and move it from Applications to the Trash.

Files it leaves behind

Preferences, a small state file with your licence key, and a plain-text log. They live in your Library folder and take a few kilobytes. Remove them by hand if you want a clean slate; keep them if you plan to reinstall, because the licence key is in there.

Nothing else

There is no daemon, no kernel extension, no login hook and no server-side account. Whatever the app changed, it changed through the standard macOS interfaces and can change back.