How to sort images so RAW files show first, before JPGs?

Added Order may determined by the Sort setting in the Import module. Try a new import with Sort = File Type in the Import module settings. Then use Sort = Added Order and Sort = File Type in the Library module. Do either of those sort settings place the JPEG files after the NEFs.

Nope.. You do have the File Type option on in Toolbar?

I don't think LrC has the sorting option you are looking for, but some comments anyway (just my own 2c):

If you "only edit the NEFs" then having the JPGs alongside these NEFs as separate images, is apparently more of a hindrance than a help to you. That is AFAICT the issue you are seeking to relieve - you are looking to push these JPGs more "out of the way".

One way is to simply filter the folder view, or collection view. to show only Raw image type. Or, if you also have got external editing files present (TIFF or PSD) to do a more complicated filter which shows those types plus NEFs but hides JPGs. This sort of thing can be saved as one or more named view filters, easily recalled for a given context using the drop-down at right end of the Filmstrip - without needing to go into Library module for the filter bar there.

Another way is to nest each JPG under its corresponding NEF using stacking. Laborious. The single advantage I can thing of for this option, would be if you sometimes wanted to work with the NEF and other times wanted to work with the JPG, and wanted both to be available for every image in a way that let you switch which one was presented and which one was hidden (when all stacks collapsed). But you've stated you ONLY edit the NEFs so probably not of interest.

So my question is, what does "extra insurance" mean for you, and how would that actually manifest in practice? Provided you have got that JPG physically on disk alongside the NEF, then the needed raw - well, non-raw - material exists to allow you to claim on that insurance. Usually we'll wait until our car gets stolen before submitting a claim for a replacement, though. We are always keen to ensure we could make such a claim, but we don't need to see that as-yet-unused replacement car always parked in our driveway. It would just get in our way for the meanwhile.

So one answer could be, make your import such that you only see the NEF routinely - and your "insurance claim" would then consist of, exceptionally, making a particular JPG become separately viewable and editable inside LrC then.

Two scenarios: you have got LrC option to [treat JPGs as a separate image] checked, but originally excluded these JPGs from import (while still ensuring they were present on disk) - your insurance claim is to just import the needed JPGs later as needed.

Or more conveniently, you have that [separate] option un-checked - producing a single "Raw+JPG" editable photo within the Catalog. This process ensures the JPG file physically accompanies that NEF file at import and also if the latter is ever later renamed or moved to another folder. Deleting would delete both, but that is what file backup is for!

You can promote a chosen NEF's "silent" JPG sidecar to a separate Catalog image in its own right either by switching LrC's setting over and selectively re-importing just that JPG - or else by leaving the setting as-is and changing that JPG's filename on disk (using OS file browser) in some way that makes LrC then no longer regard it as a "silent" sidecar for purposes of a new selective import. The Raw+JPG pairing only happens when both files are identically named apart from their file type extension.