Idea for set up structure

setup.php

<?php

namespace themename\setup;
//todo: abstract actual values to another object and pass it to this contructor
class MainThemeSetupMaster
{
    private $cssSetup;
    private $jsSetup;
    public function __construct()
    {
        $this->cssSetup = new CssSetup();
        $this->jsSetup = new ScriptSetup();
    }
    public function initScripts()
    {
        $this->cssSetup->init();
        $this->jsSetup->init();
    }
}

class CssSetup
{
    public function init()
    {
        // call privates
        add_action( 'wp_enqueue_scripts',
            function ()
            {
                $this->_theme_styles();
            });
        add_action( 'wp_enqueue_scripts',
            function ()
            {
                $this->_vendor_styles();
            });

    }
    private function _theme_styles()
    {
        // queue theme styles
        
    }
    private function _vendor_styles()
    {
        // queue vendor styles
    }
}
class ScriptSetup
{
    public function init()
    {
        // call privates
        add_action( 'wp_enqueue_scripts',
            function ()
            {
                $this->_theme_scripts();
            });
        add_action( 'wp_enqueue_scripts',
            function ()
            {
                $this->_vendor_scripts();
            });

    }
    private function _theme_scripts()
    {
        // queue theme scripts
    }
    private function _vendor_scripts()
    {
        // queue vendor scripts
    }
}

Looks good.

One thing I'd probably do slightly different is start with all of those classes being just one class that handles all the CSS/JS/etc. and then only split them out into different classes if they become too complex. Otherwise you have to switch context a lot between the different classes. If you do that, you'll find that it cuts the size of the current file at least in half.

Cool thanks. I am going to refactor it.