The “(more…)” link gets filtered through the_content_more_link
in get_the_content()
. You can hook into this function and let it return whatever you want.
add_filter('the_content_more_link', 'more_button');
function more_button($more_link) {
return '<button class="readmorebtn" onclick="' .
esc_attr('window.location="' . get_permalink() . '"') .
'">Read more</button>';
}