You need to use L rather than S-l.
This is a quirk of key sequences in Emacs -- for letters (a-z), the shift modifier syntax S- won't do what you want, unless C- is also used in the sequence.
You should therefore write upper-case letters explicitly in key sequences.
See also https://stackoverflow.com/q/38180797/324105
You can give SPC and SPC-b a description for which-key by using the :ignore keyword:
:states '(normal visual insert emacs)
"" '(:ignore t :which-key "description for SPC") ; <<< added
"b" '(:ignore t :which-key "description for b") ; <<< added
Probably depends on your keyboard. I have a typical US PC keyboard. I don't know whether I have a <menu> key (for C-<menu>). But the <next> key is the key labeled Page Down.
To see how Emacs calls any given keyboard key, use C-h k. For example, if I use C-h k and hit the key labeled Page Down, Emacs describes what it calls key <next>....
First one needs introduce <deletechar> to command mode of xah-fly-keys (XFK):
"Modify keys for xah fly key command mode keys
To be added to `xah-fly-insert-mode-activate-hook'"
(define-key xah-fly-key-map (kbd "<deletechar>") 'ss-xfk-delchar-keymap)
;; more here
Then add the needed ...
For reasons @Drew pointed out, this is not necessarily a good idea. However, if you really want to do it, you can simply bind exit-minibuffer to M-x in the minibuffer-local-map which is the keymap used when in the minibuffer.
You can find information on minibuffer key maps here: https://www.gnu.org/software/emacs/manual/html_node/emacs/Minibuffer-Maps.html
Per the conventions referenced in a comment on the question,
Sequences consisting of C-c and a letter (either upper or lower case) are reserved for users; they are the only sequences reserved for users, so do not block them.
So as a user, you redefine arbitrary keys outside of that range at your own risk, though of course people do it all the time.
A little experimentation shows that :bind uses the global keymap by default. You can define C-z as a prefix by creating a keymap and adding it as the definition of C-z to the global keymap. Here I create a sparse keymap, since only a few entries are needed:
(define-key global-map (kbd "C-z") (make-sparse-keymap))
After you have defined C-z as a prefix this ...
Similar to How do I turn-off smartparens when using prelude?, the following might do it:
(defun rebind-keys ()
(define-key smartparens-mode-map (kbd "M-s") nil)
(add-hook 'smartparens-enabled-hook 'rebind-keys t)