only redirect https://example.com/demo/index.php to https://example.com/

If you only want to redirect that exact URL, without a query string then you need to reverse your existing rules and include an additional condition that checks that the QUERY_STRING is empty. The RewriteRule only matches against the URL-path, which excludes the query string.

You are also missing a space before the flags argument in the first rule.

For example:

# Redirect exact URL to home page
RewriteCond %{QUERY_STRING} ^$
RewriteRule ^demo/index\.php$ / [R=302,L]

# Redirect other URLs with query string OR path-info
RewriteRule ^demo/index\.php /demo/index.php?m=page [NC,L]

You don't need L and END.

Test with 302, and change to 301 only once you have confirmed it works OK. To avoid caching issues.

You may use these rules in site root .htaccess:

RewriteEngine On

RewriteCond %{THE_REQUEST} \s/+demo/index\.php\s [NC]
RewriteRule ^ / [L,R=301]

RewriteRule ^demo/index\.php$ $0?m=page [QSA,NC,L]

This was created by htaccess redirect generator I use very often.

RewriteCond %{QUERY_STRING} ^$
RewriteRule ^demo/index\.php$ https://example.com/? [R=301,L]

Maybe "?" sign is redundant because of empty query string match.

Not sure, but You also can try to add

RewriteCond %{REQUEST_METHOD} !=POST 

in case You can't log in. All together:

RewriteCond %{REQUEST_METHOD} !=POST
RewriteCond %{QUERY_STRING} ^$
RewriteRule ^demo/index\.php$ https://example.com/? [R=301,L]