First of all: Lineage OS for the FP2 is now at Android 8.1 (and there’s also already a test version of Android 9), so still Lineage is ahead in terms of the major android version.
The one post you provided about stable vs. nightly refers to CyanogedMod (in short: the predecessor of Lineage). I’m not 100% sure but I think Lineage doesn’t distinguish between stable and nightly but between official and unofficial. Not sure if the distinctions are the same. But according to their FAQ, to be come “official” the build has to be stable.
Their infrastructure to release builds is still capable of doing nightly builds (and AFAIK they are all called “nightly”), but effectively for the FP2 new builds get released weekly.
But it’s no problem to skip one if you like. I for example only update when the new build contains the monthly security patches.
There is one issue with FPOOS (https://bugtracker.fairphone.com/project/fairphone-android-7/issue/56) that led to a regression between Android 6 and 7. In effect FP(O)OS at the moment is not compatible with Instagram and Pokemon Go.
I don’t use those but on Lineage I was able to install Instagram which isn’t the case for FP(O)OS.
If you don’t feel comfortable installing LOS, maybe there is a meetup or a Fairphone Angel near you where you could get assistance.