The nicki build is now available - see this post for the link
You are correct - /e/OS is built on LineageOS (from Pie onwards) or its predecessor CyanogenMod.
The reason these devices are “stuck” on nougat is that Lineage stopped supporting them when Pie (lineageOS 15.1) came along, probably because they used an old kernel that couldn’t be upgraded or ported to Pie. So the latest available Lineage to build on is actually CyanogenMod 14, which means the latest /e/ you can get is Nougat.
I don’t know to be honest. I’ve only tested the nougat builds on Xperia Z5 Compact, bcause that’s the only one of these devices I own, and even then, I just give it a basic sanity check to see that it boots, can access the internet, make calls and send texts.
I did notice with this build that the Mail app is a bit flakey: it kept crashing and I had to clear its storage and re-enter account details. Anyway I usually recommend using K-9 instead of /e/'s app (which is a fork of K-9, lags behind it in terms of bug fixes and new features, and any updates of /e/'s app won’t make it to the nougat builds).
Also, I don’t know if Apps (the app store app) has got any updates recently. Again, I recommend F-Droid and Aurora Store for installing and updating apps