Nexerelin adds a lot, so it might not be for you yet, but it makes Starsector more like a 4x game, with invasions, diplomacy, spies(agents), and some other stuff.
On the faction side a good introduction mod is Scalartech Solutions.
They are a fast high-tech faction, with some good weapons, and fitting their ships isn't too complicated.
For a expansion of vanilla ships, Ship and Weapon pack has lot's of good vanilla-adjacent ships, but similar to Nexerelin it is a lot for newer players. Another good mod would be Torchships and Deadly Armaments, which is full of nice balanced ships.
For most mods, you'll need to also download and activate these libraries, or the other mods won't work:
GraphicsLib:
https://fractalsoftworks.com/forum/index.php?topic=10982.0(Also for this one, you might want to change the settings of the mod, because this adds all kind of graphics and lighting and stuff. Here's a link to my guide for that
https://fractalsoftworks.com/forum/index.php?topic=19597.0)
LazyLib:
https://fractalsoftworks.com/forum/index.php?topic=5444.0MagicLib:
https://fractalsoftworks.com/forum/index.php?topic=13718.0Finally, for stupid, unbalanced, but really fun, there's Kingdom of Terra. This faction whacks, smacks, and pushes other ships, instead of just using weapons. If that isn't enough, there is a ship that is a giant T Rex skull, that chomps ships.