Glad that you got most of it out! That's real progress given where you started! It's amazing how much of an improvement fret leveling makes, I have found. I have only had one neck that didn't need at least some leveling. The one that didn't was a Warmoth radiused neck. I'd expect that from a neck that sells for more than any kit I have bought ;-)
Getting rid of the buzz with normal playing is pretty good. Eventually you will hit the point of diminishing returns trying to get every bit of buzz out of the low E when you are playing hard. You may want to see if you can hear it when amplified before doing too much more.
If you do decide that you want to try to get that last bit out, you might use one of these on the 2nd fret:
Attachment 45706
These are sometimes marketed as "fret dressing sticks" or "fret dressing tools" in places that sell to guitar people. The exact same item is often sold cheaper at wood working sites where it is called a "sanding detailer stick."
Something nice about these is that you can use them to lower a fret just under the string that is buzzing without taking more metal off the whole fret. I have used mine on the low end of the fretboard when the fretboard is pretty much level, but I have a small, persistent buzz, usually one the 2nd or 3rd fret, and just one one or two strings.
If you decide to go after 16-22. Tape off the frets from 1-11. Put two pieces of blue painters tape on the 12th fret. Then level the frets from 13-22. The painters tape will raise the height of the 12th fret just enough to take off just a bit of the frets below that. You could concentrate just on the bass side of the neck if you want, or the whole neck. If you want Either way that should eliminate your problem at the treble end of the neck.
I just did this with a Tele neck, although I raised the 9th fret rather than the 12th for a slightly more gradual slope. I have done it both ways and don't think it makes much difference, since you only have a little buzz you could do it either way.
Hope this helps. It always seems to me that the last 10% involves 90% more effort ;-)