I noticed the ray 3.14 model intentionally adding things into my videos at the end to intentionally make it unloopable.

I’ve made a few looping GIF files using Firefly, but I can’t remember which models I used.  I remember that one had a tiny glitch because the first and last frames were identical, so it appeared to pause for a moment.  Deleting a frame fixed it.  Another one needed several frame removed to get a clean loop.  I run the video through Handbrake to reduce the frame rate to 25 or 15fps and then Export > Save for web as usual to make it a frame animation.  There is a 5mb file size limit on this forum, so when I want to share here, I have to clobber the heck out of it to get it to fit.  I think this might only have 16 colors!  I block up the black point to win back some contrast when I ned to get the file size this low.

I’m sorry, I can’t remember what I used.  I just use what works.  I’ve just remembered this one.  The GIF is 8MB so I can’t share it here, but the issue was the young children.  IIRC most of the Firefly models will baulk when asked to create video with youngsters in a reference image, so that was what forced thew model I used.

Another trick I use a lot is making the subject dance by copying the clip and reversing it, like in this video of my wife’s granddaughter.  

So, all I can say is try different models, and be prepared to edit to fine tune.  I used Premiere Pro, but I hear that the Firefly editor is very good.