PPS - Looking at your image on my big desktop machine /monitor instead of on my tiny cell phone, I now see that there is both fine and coarse banding. This is just too much to fix by any of the techniques we've been discussing.
Either drop in a completely new sky background, or, as I said earlier, even better, get a new scan done by a reputable lab. If you can get some money out of the old lab, do so, but don't accept any offer by them to re-do the scan for you. If their scanners are in such bad shape, there's a good chance they will damage your negative. Walk as fast as you can away from them.
If, by some chance, these problems really are on the negative, then drop in a new sky.
T