Ah, I hadn't heard of LineageOS. That looks great. However, it would appear that LineageOS, while itself unrooted, does need device-rooting for actual installation:
http://www.androidbeat.com/2017/02/how-to-install-lineageos-android-device/ [in the Prerequisite Section]
If I ever did get this thing rooted, LineageOS might solve the second problem of missing custom ROMs.
Thanks for the heads-up,
Jake