To everyone like me who wishes there was a working mod to get stars in the core of your galaxies since the last update (3.6) seemed to have broken that part of this mod -- you can change some values in the files for galaxy generation yourself, all you need is something simple like Notepad.

You want to go to this file location inside your Steam directory, which for me was something like this:

Steam\steamapps\common\Stellaris\map\galaxy

and open the file named galaxy_shapes with your preferred text editor (Notepad works just fine). Inside is a list of all vanilla galaxy shapes and a few settings pertaining to each. Go to whatever shape you want to use, and we're focusing on these settings:

num_stars_core_perc
stars_min_dist

The value of the first setting is the percent of stars in the galaxy that spawn in the core, default for all shapes is 0. The higher you change this value, the more stars you'll have in the core and the less that will be outside. The second setting is some -minimum- game unit distance between stars. Note that the only usual observable change with this setting is if you have stars spawning in the core.

I changed num_stars_core_perc to 0.18 and stars_min_dist = 5.0 and I think it's a decent enough approximation for what this mod does. Be careful about increasing num_stars_core_perc alongside decreasing stars_min_dist, because you could over-pack your core (and make a really sparse rest of your galaxy) which might not be preferable or stable.