I am using the Logitech MX Keys which is great.
As Sleep exists as a menu command, you could make your own…
System Prefs > Keyboard > Shortcuts > App Shortcuts
Select All Applications, press the + button, type Sleep then pick a key command - I'd pick something hard to do by accident, like maybe Cmd ⌘ Opt ⌥ Ctrl ⌃ Shift ⇧ S
I used D in the example image, because I already have S used for Shutdown