Best App for Slideshows

I am a professional photographer and would like to offer my clients slideshows of their images. I am a Mac user but neither iMovie nor Lightroom offer anything special  . I don't need super fancy but something a little bit more than the plain slideshows you can create in LR.  I am looking into After Effects but unfortunately I have to pay $25 every time I buy a template for each of my clients. I can create my own I guess but if I am going to put the effort into learning how to create templates etc. I was hoping to first figure out which App is the best for Slideshows.  (Note: I own Photoshop, Lightroom, Premiere Pro (which seems to be a little bit over the top for simple slideshows). I don't mind paying for another app but it has to be simple to create as I need at least 15 slideshows a month. Thank you!