Ok, excuse the double post, but I sorted this and I wanted to give an update for anyone who is stumbling across the same issue so:
I've edited the phpBB_to_Flarum.php file to accommodate my needs and on line 178 after the text is properly formatted to work with Flarum ($postText = formatText($exportDbConnection, $post['post_text']);) I have added my modifications (to simply replace all the html tags that were causing issues because of unclosed tags) and simply reformatted it to work again with the Flarum XML structure
$postText = str_replace('<p>', '', $postText);
$postText = str_replace('<r>', '', $postText);
$postText = str_replace('</p>', '', $postText);
$postText = str_replace('</r>', '', $postText);
$postText = str_replace('<br/>', '', $postText);
$postText = str_replace('<e>', '', $postText);
$postText = str_replace('</e>', '', $postText);
$postText = str_replace('<E>', '', $postText);
$postText = str_replace('</E>', '', $postText);
$postText = '<r><p><r>'.$postText.'</r></p></r>';
Not the prettiest solution, I'll admit, but it does the job and migration went through successfully 😅
Thank you so much @luceos , stupidly enough talking this through helped me a lot lol
