Don't use an elaborate template for the slideshow. These are fixed and images are moved around to fit the template. Stick to the Ken Burns or the Classic and you should be okay.
Alternatives to Photos slideshow include:
iMovie is on every Mac sold.
Others, in order of price:
PhotoToMovie $49.95
PhotoStage $60/$80 (depending on the version you prefer)
FotoMagico $99
Final Cut Pro X $299
Other folks have good results with apps like Keynote or Powerpoint.
It's difficult to compare these apps. They have differences in capability - some are driven off templates. some aren't. Some have a wider variety of transitions. Others will have excellent audio controls. It's worth checking them out to see what meets your needs. However, there is no doubt that Final Cut Pro X is the most capable app of them all. You get what you pay for.