Cannot modify header information -headers already sent (Wordpress Custom Plugin) [duplicate]

I'm creating a plugin for a small business. I need the plugin to do a few things:

• Deactivate + delete pre-installed plugins
• Install Kadence (including kadence theme, child theme and kadence blocks)
• Removing default posts e.g. 'Hello World!'
• Remove old themes e.g. Twenty Twenty Four
• Add plugins such as SVG support

For the most part the plugin works, however when I try to install and activate a plugin I get an error of ob_end_clean(): Failed to delete buffer. No buffer to delete in /var/www/htdocs/wp-admin/includes/plugin.php on line 740 and Cannot modify header information - headers already sent by (output started at /var/www/htdocs/wp-includes/functions.php:5420) in /var/www/htdocs/wp-includes/pluggable.php on line 1435.

All the functions are ran in the plugin's activator file as I need everything to run when the plugin is activated.

I'm using a custom plugin that has been built via https://wppb.me/

Here is the code I'm running when installing Kadence:

    public static function add_kadence() {
        include_once(ABSPATH . 'wp-admin/includes/class-wp-upgrader.php');

        $kadence_blocks_slug = 'kadence-blocks';
        $kadence_theme_slug = 'kadence';
        $child_theme_slug = $kadence_theme_slug . '-child';
    
        // Theme installation
        $kadence_theme_installed = wp_get_theme($kadence_theme_slug);
        if (!$kadence_theme_installed->exists()) {
            // Theme URL
            $kadence_theme_url = 'https://downloads.wordpress.org/theme/' . $kadence_theme_slug . '.zip';
    
            // Create instance of theme installer
            $kadence_theme_installer = new Theme_Upgrader();
    
            // Install Theme
            $installation = $kadence_theme_installer->install($kadence_theme_url);
            if (is_wp_error($installation)) {
                error_log('Kadence theme installation failed: ' . $installation->get_error_message());
                return;
            }
        }
    
        // Child Theme installation
        $child_theme_installed = wp_get_theme($child_theme_slug);
        if (!$child_theme_installed->exists()) {
            // Child Theme URL
            $kadence_child_theme_url = 'https://dl.dropboxusercontent.com/scl/fi/zca2facxnwbl3le9jg6ke/' . $kadence_theme_slug . '-child.zip?rlkey=fo2sjq4fbmwhctb677u559w2a&dl=0';
    
            // Create instance of theme installer
            $kadence_child_theme_installer = new Theme_Upgrader();
    
            // Install Theme
            $installation = $kadence_child_theme_installer->install($kadence_child_theme_url);
            if (is_wp_error($installation)) {
                error_log('Kadence child theme installation failed: ' . $installation->get_error_message());
                return;
            }
    
            // Switch to Child Theme
            switch_theme($child_theme_slug);
        }
    
        Plugin installation and activation
        $kadence_blocks_installed = is_plugin_active($kadence_blocks_slug . '/' . $kadence_blocks_slug . '.php');
        if (!$kadence_blocks_installed) {
            // Plugin URL
            $kadence_blocks_url = 'https://downloads.wordpress.org/plugin/' . $kadence_blocks_slug . '.zip';
    
            // Create instance of the plugin installer
            $kadence_blocks_installer = new Plugin_Upgrader();
    
            // Install Plugin
            $installation = $kadence_blocks_installer->install($kadence_blocks_url);
            if (is_wp_error($installation)) {
                error_log('Kadence Blocks plugin installation failed: ' . $installation->get_error_message());
                return;
            }
    
            // Activate Plugin
            activate_plugin($kadence_blocks_slug . '/' . $kadence_blocks_slug . '.php');
        }
    }

This is ran within the activate function using self::add_kadence();.

Edit: I have read through similar questions but none are regarding custom plugins within wordpress and doesn't help solve my problem.