that's right.
you know, it has been so long since I used a stable build I can't remember. Try a stable sysupgrade image - the UI won't let you do something silly without a warning.
The r7800 is almost identical to the r7500v2. @hnyman maintains a "community build" for the r7800 and I believe I've seen that you can go back and forth between factory and sysupgrade images for these r7800 images (you can not use any r7800 image on the r7500v2 tho).
I find following just about any thread for the r7800 like this one helpful.
Good forum etiquette is to respect the 'OP's' scope for the thread (i.e. @hnyman generally requests that questions on his community forum thread only be relevant to his r7800 builds) but the other threads are a little more forgiving if your question is applicable to the other devices. @hnyman is really good and approachable so don't be too shy.