Get current directory name in htaccess and prevent redirection

I have this .htaccess file:

Options +FollowSymLinks
DirectorySlash On
RewriteEngine On

# Remove trailing slash for non directories
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^(.*)/$ /mydir%1/$1 [R,L=303]

# Make direct files accessible
RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_URI} !^base.php.*$
RewriteRule ^([^\n]+)|(.*)$ base.php?___ROUTE=$1 [QSA,END]

Code (markup):

Which having following redirection behaviors:

1. ✔️ d.com/mydir → d.com/mydir/
2. ✔️ d.com/mydir/nothing → run
3. ✔️ d.com/mydir/nothing/ → d.com/mydir/nothing
4. ❌ d.com/mydir/exist_dir → d.com/mydir/exist_dir/?___ROUTE=exist_dir
5. ❌ d.com/mydir/exist_dir/ → run
6. ✔️ d.com/mydir/exist_file.css → run
7. ✔️ d.com/mydir/exist_file.css/ → d.com/mydir/exist_file.css

By run I mean It respond file contents or execute base.php?___ROUTE=$1

Now I have a problem and a question

Problem:

I don't know how to fix redirection number 4.

Number 5 should redirect to d.com/mydir/exist_dir.

And number 4 should execute base.php?___ROUTE=$1 instead of redirection

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Question:

How can I use current directory name in .htaccess instead of writing it's name in each project?

I mean /mydir in RewriteRule ^(.*)/$ /mydir%1/$1 [R,L=303]