As far as specific recommendation(s) go:
CONTENT Nexerelin - Dramatically expands on the campaign layer, making things much more dynamic
- Can definitely result in a very difficult early game depending on your starting conditions
Ship and Weapon Pack-Adds ships, weapons, as well as unique ""boss"" ships
Tahlan Shipworks- Similar to Ship and Weapon Pack, but flashier. Lots of additional goodies are added that you can come across during exploration. Also adds the Legio Infernalis, a ""boss"" faction. (Although, they probably only really get up to things with Nex)
Secrets of the Frontier - Adds new combat objectives, such as defensive emplacements or objectives that reinforce your ships with drones over time. Also one of the few mods with quest content
QOL Stellar Networks - Basically, uh...space google? This makes it easier to find specific hulls, fighters, or weapons you're looking for. Honestly worth it just for the freaking contact calling feature.
Flux Reticle - replaces the default cursor during combat with a reticle that displays flux level.
It also allows you to toggle "strafe and turn to cursor" behavior during battle by pressing "K"
MISC Combat Chatter- ships make various statements based on their current condition (being overloaded, running out of missiles, etc)
GraphicsLib- Probably will need it eventually,especially if you want to go mod heavy
LazyLib- Ditto
MagicLib- Ditto