The space limit on a single-layer BD is somewhere around 23.5 GB. 17.9 as-is without any extras is fast approaching that.
I suppose you could start dropping selected audio (particularly lossless audio) to create more space, then of course the revised menus.
There wouldn't be any layer break on a BD-25, of course so no worries there. Even if there were, the software would handle that seamlessly.