Try binding keys also in isearch-mode-map. These keys are among those bound there by default, for example:
M-s r isearch-toggle-regexp
(define-key isearch-mode-map (kbd "C-s") '...
The following setting makes the left alt/option key behave like a system default key (so that one may use special symbols, foreign language characters, and so forth), and the right alt/opion key behaves as the Meta key:
(setq ns-alternate-modifier 'none
Just found this issue, looking around the internet I came across this page, and the solution is at the bottom, basically you have to set the keyboard mapping with the following command on the Debian terminal:
setxkbmap us -variant intl
I use US-INTL, so from what I am reading on your question, this is your case too.
Finally you might want to add it to your ....
In non-windows mode, changes to ns-* variables (such as ns-alternate-modifier and ns-command-modifier) are ignored. In particular, if the -nw option is passed as argument to emacs, the logic that propagates changes to ns-* variables (implemented in src/nsterm.c) is not evaluated. See the call to init_display in the src/emacs.c in the Emacs 27.1 source code. ...