"Neutron star = nucleus" is very elided. The crust of neutron stars contains ordinary elemental matter with a density around that of a white dwarf (specific density of ~10⁶), but descending down to the core it explodes to ~10¹⁴, which is nucleus density and a little beyond.

There's also the whole menagerie of nuclear pasta [1] where gnocchi and spaghetti you could maybe consider "nuclei" unto themselves. There's also waffles & lasagna, but when you get into the anti-spaghetti and anti-gnocchi realms, then you're into the "one nucleus" region.

[1]: https://doi.org/10.1103/PhysRevC.96.025803

A neutron star is about 20% protons.

Really? Closer to the surface I imagine?

Just let a single proton loose in the vicinity of a pure neutron star, wait to be attracted to it and tada, we made a super heavy and stable hydrogen isotope!

I do not believe that it would be stable. The proton would convert to a neutron and release an electron. If it were stable for the neutron star to have protons in or on it, then it would already have some.