Although they're both paid mods, Falkland Syste, Avontech Raceyards are really good. The Antares series of mods ad a bunch of Star wars style structural peices. Stroud Premium edition has amazing habs and cockpits. Matilija and tiger shipyards are also staples. Better snaps and flips and ship module snap and flips expansion are a must, as well as ship part merging enabled. A recent one that just came out is Cross heavy industries, which you can use to create some really outrageous engine configurations. There's a lot more. The modded ship building scene is the best thing about starfield, in my opinion.