Emacs: 1 Key for M-x
The default key for execute-extended-command
is Alt+x.
On Linux, ▤ Menu key does execute-extended-command
by default.

You should set F8 or ▤ Menu key for it. Single key is easier to type than Alt+x.
For how, see: Emacs: Bind Menu/App Key.
Turn On icomplete-mode
When Alt+x execute-extended-command
, you can press the Tab key to show a list of suggestions.
You can turn on icomplete-mode
. It'll show a list of completions automatically.