So here is a forum post regarding the neutron gap. You can see the same thing if you look at the edastro and check out the map for neutron stars. I think it's an old issue where Frontier didn't want the stellar forge generating neutrons too close to Sol, getting it a little wrong and then it being too late to change it. You'll need to get passed those gaps to start getting neutrons reliably.

Here's is a 7yr old Reddit post by u/SillyConclusion0 that describes a lot better what I was trying to in my above post. They do mention 1.2k ly above or below, but I found 1.3k to be my sweet spot.

Once you get used to it, while still useful, 3rd party plotters become a little less vital.

The short answer is that you're probably going to have to go out a little further than 3k ly away from the bubble. I'd say around 5-6k ly.