Hi monmidt,
If you make direct changes in the code then while updating to new version your customized code will be lost. So better is making language translation file. If you wanna direct change then each and every time you update to new version you need same customization.
The readmore file text is inside theme folder(interface)/inc/structure/content-extensions.php on line 173, 511, 637, 742 and
theme folder(interface)/inc/widgets/interface_widgets.php on line 289
Hope this may help you
Thank you!