You are using the_post_thumbnail() which will echo the actual thumbnail. You should use get_the_post_thumbnail() if you want the value to be returned, not echoed.
So, change:
$slide_img = the_post_thumbnail();
To:
$slide_img = get_the_post_thumbnail();
You should also, as Matt points out, make sure you are using the correct meta field names and check if the values are set correctly.
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