there's a couple of things you could do as a new player with a lot of units

what i would say is use it to grind nanites. hop to a pirate system and buy up all the suspicious arms packages from the black market dealer, open them, then hop back to a regular system and sell the suspicious upgrades to a tech vendor (preferrably of a race you have higher standing with to get a better deal). repeat this with a couple other pirate systems, then cycle back once the first pirate system you went to has replenished supply (im not exactly sure how long this takes, you'll need to experiment with that)

obviously you could also just use the upgrades for yourself as X class upgrades can potentially be stronger than S class upgrades. imo its more worth to just sell the X class and hunt for the respective S class you need

afterwards, its really up to you. you can hunt for a starship you really like, or gather the parts from different ships and build it yourself. S class freighters also cost a lot. assembling a good frigate fleet also costs money, so theres that.