Photos moves the images around to fit the template you have chosen. Using Classic or Ken Burns should reduce this issue. I have no idea about the transitions.
In terms of a replacement: do you really want to replace iPhoto/Photos entirely just for this one feature? As in migrate to an entirely new photo manager? Have you considered just exploring slideshow apps? What's your budget? What size is your Library? How much does it grow per annum?
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
Others use 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.