Set image alt field on upload and on use - https://www.damiencarbery.com/2018/07/set-image-alt-field-on-upload-and-on-use/

image-alt-field.php

<?php
/*
Plugin Name: Set Image Alt field
Plugin URI: https://www.damiencarbery.com
Description: Set the alt field of uploaded images to the title. Copy image title to alt when inserted into editor. Set alt of post thumbnail to post title. Code copied from various sources.
Author: Damien Carbery, Francisco Ruiz, Dan and Bojan Petrovic.
Version: 0.1
*/


function dcwd_title_to_words( $title ) {
	// Sanitize the title:  remove hyphens, underscores & extra spaces:
	$title = preg_replace( '%\s*[-_\s]+\s*%', ' ',  $title );

	// Sanitize the title:  capitalize first letter of every word (other letters lower case):
	$title = ucwords( strtolower( $title ) );

	return $title;
}


// Copied from: https://brutalbusiness.com/automatically-set-the-wordpress-image-title-alt-text-other-meta/
add_action( 'add_attachment', 'dcwd_set_image_meta_upon_image_upload' );
function dcwd_set_image_meta_upon_image_upload( $post_ID ) {

	// Check if uploaded file is an image, else do nothing
	if ( wp_attachment_is_image( $post_ID ) ) {

		$my_image_title = get_post( $post_ID )->post_title;
		$my_image_title = dcwd_title_to_words( $my_image_title );

		// Create an array with the image meta (Title, Caption, Description) to be updated
		// Note:  comment out the Excerpt/Caption or Content/Description lines if not needed
		$my_image_meta = array(
			'ID' => $post_ID,			// Specify the image (ID) to be updated
			'post_title' => $my_image_title,		// Set image Title to sanitized title
			// Damien: Omit setting the caption as I rarely use captions when I insert images.
			//'post_excerpt' => $my_image_title,		// Set image Caption (Excerpt) to sanitized title
			'post_content' => $my_image_title,		// Set image Description (Content) to sanitized title
		);

		// Set the image Alt-Text
		update_post_meta( $post_ID, '_wp_attachment_image_alt', $my_image_title );

		// Set the image meta (e.g. Title, Excerpt, Content)
		wp_update_post( $my_image_meta );

	} 
}


// Enhanced version of: https://wordpress.org/plugins/automatic-image-alt-attributes/
add_filter('image_send_to_editor', 'dcwd_auto_alt_fix_1', 10, 2);
function dcwd_auto_alt_fix_1($html, $id) {
	$image_title = get_the_title( $id );
	$image_title = dcwd_title_to_words( $image_title );

	return str_replace('alt=""','alt="' . $image_title . '"',$html);
}


add_filter('wp_get_attachment_image_attributes', 'dcwd_auto_alt_fix_2', 10, 2);
function dcwd_auto_alt_fix_2($attributes, $attachment){
	if ( !isset( $attributes['alt'] ) || '' === $attributes['alt'] ) {
		$attributes['alt'] = dcwd_title_to_words( get_the_title( $attachment->ID ) );
	}
	return $attributes;
}


// From: https://mekshq.com/change-image-alt-tag-in-wordpress/
/* Replace alt attribute of post thumbnail with post title */
add_filter( 'post_thumbnail_html', 'meks_post_thumbnail_alt_change', 10, 5 );
function meks_post_thumbnail_alt_change( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
 
	$post_title = get_the_title();
	$html = preg_replace( '/(alt=")(.*?)(")/i', '$1'.esc_attr( $post_title ).'$3', $html );
 
	return $html;
 
}