How many photos? Dozens, hundreds, thousands?
Are they all in one folder, all at the root of the external disk, scattered among many folders, what?
Are you still running macOS 10.12 ("Sierra") on your MBP 13" as noted in your profile?
If they are all in one place (either a folder or the root of the drive) or you can settle for viewing one folder at a time, I can suggest two simple, free ways. They both work in Mojave or Catalina; I think they both worked earlier, but maybe not back in Sierra.
You could use Quick Look. Click on the first photo you want to see, then hit the space bar. That photo should be visible, enlarged to nearly the full screen. Use the up arrow key to advance to the next photo (according to whatever order is chosen for the folder); use the down arrow key to go backward. Use the space bar or escape key to exit.
You could use Preview on blocks of pictures. I wouldn't do as many as a thousand at a time, but a hundred should be OK. Select a hundred photos, right-click (or control-click) on the first one, select to open with Preview (which may be the default), then use up/down to scan through them. Close the window or quit Preview to exit.
If you are looking for something more built for the specific purpose of viewing as a slide show, you may need to find an application in the App Store. The one I use is overkill for that (as it serves many other functions), but there are many shown in a quick search there.