I’ve found the line to modify in the Slideshow Joomla particle
line 552 <a href="{{ article.route }}" target="{{ particle.buttontarget|e }}" title="{{ display.title.limit ? article.title|truncate_text(display.title.limit) : article.title }}" rel="noopener noreferrer">{{ display.read_more.label|default('Read More...') }}</a>
