I've put a fair amount of hours into all 3 of the games you mentioned; StarSector, Star Valor, and Endless Sky (as well as its creative predecessor EV & EV Nova). For full disclosure, there are aspects of each game I like, and "deal breakers" for me that keep me from really being invested in the game.
StarSector will definitely fill that itch you have of traveling between systems, trading, mission running, upgrading your ships, building new ships from scratch, etc. Vanilla, maybe not a perfect match. But the modding community we have is phenomenal.
Yes you can build your own ships, once you get your own colony and provided you've got the blueprints on how to build whatever you want to build (kind of like Eve). Near everything is customizable, every weapon mount you can choose what sort of weapon you want to put in; a big gun you can only fire every so often, or a smaller gun that can fire endlessly. And your ships have OP which can be spent on weapons, energy systems, or hullmods which do things like increase speed, increase armor, etc. It is not nearly as customizable as Endless Sky or Star Valor though; SS ships are mainly pre-fab, you aren't given the option of getting an empty hull and putting in your own engines, armor, power systems, shields, etc. like you are in the other games you mentioned.
You want trading? Vanilla has a few missions for that,
Space Truckin will give you up to a pure dedicated play-style of cargo running for credits. I'll note vanilla "trading" isn't at all like SV/ES; buying a commodity from planet A and hauling it to planet B generally isn't profitable due to tarrifs, you've really got to find large price discrepancies across systems, and then make trades on the black market subject to the planet's faction becoming hostile to you for not paying taxes. That's where Space Truckin helps with dedicated "please move my cargo from A to B for $$$" mission that are always available.
You want mission running? Vanilla has a few, there are a few mods like
More Military Missions that will add the sort of missions I see in Endless Sky like planetary defense, escort cargo freighters on a trade run, etc.
You want lore? Alex has a good Hegemony-TriTach-Church galaxy / political can of worms which is akin to the Free Worlds-Republic-Syndicate plot-line of Endless Sky. And I can't even name & link all the content expansions the community has made, but Junkyard Dogs, Carter's Freetraders, Underworld, Roider Union, Hazard Mining, Phillip Andrada, UAF, and others are frequently ones I add into my playthrough for more factions, more lore, more fun.
You want battle? The battles in SS are significantly better in my opinion than the other games you mentioned. Fleet on fleet engagements with tactical deployment. Not the sort of "you were peacefully doing your own thing and some enemy ship warped right into the middle of your fleet" sort of stuff from the other games.
In fact, one of the ways I think StarSector is a better play style than SV or ES is because engagements are fully contained and you get a reasonable amount of loot.
One of my biggest grips playing Endless Sky is you've got to fight some massive battle with pirates or whoever, and then all the ships you disabled are ready for you to loot. And yeah, it is a full loot system; you disable a ship and you've got the opportunity to recover the ship and all its gear & cargo. But as soon as you're done fighting, another wave of pirate scum flies in (or worse, some Republic or Free World do-gooder), destroying those ships YOU disabled, and leaving you with zilch for your efforts. And then this repeats ad nauseam until you give up. So it's full loot, but more like
fool loot, as you're lucky if you can get to your battle prize before some idiot destroys it all.
StarValor is little better; you kill a giant ship and your loot is what? A single weapon? A shield generator? Plus a small handful of credits? That's like Wastelands where you kill some heavy machine gun full body armor clad guy and your battle spoils is a pack of playing cards worth $5, all that gear magically vanished.
In SS, you fight an engagement, the battle is over, time is basically paused while this all goes on, and your looting efforts are done immediately, giving you a fair amount of the goodies of what you just destroyed. You kill stuff, you get loot, you get to move along. Nobody magically appears out of nowhere to harass you and make you leave all the loot behind. Think of it as similar to Mount & Blade if you've played that at all.
Tying into that, I detest how both SV & ES have "magical spawns". New enemies appear from nowhere, and often magically just after you've finished kicking their friends' butts. ES is the most egregious in this; you jump from system A to B, there were no pirates ANYWHERE in system A, but magically as you enter system B a fleet of half a dozen flies in on top of you, entering from system A's hyperspace lane that you just exited from....how exactly??? In SS, fleets spawn, but they have to live in reality. Kick a guy's butt, and his friends maybe get so angry they send a hit fleet after you. But that fleet doesn't just magically teleport in via a wormhole or drop out of hyperspace right on top of you. They launch from their faction's planet, travel in space at the same speed as you, and have to work their way towards you. You have time to adapt; do I turn off my electronics and try to hide from them all, waiting for them to give up? Do I run towards a friendly fleet or station and get their help defeating this threat coming at me? Do I goad them into chasing me, hide in an asteroid belt, and then pick off their straggling support fleets one by one until the main fleet is outmatched? Or am I tough enough that I can solo their whole horde at once?
You mentioned a dislike for travel time. There are some speed up mods in SS that will help, but if you're going on an exploration type mission, you can expect to spend an hour flying out to the extremes of space, avoiding dangers along the way, searching out loot, getting into fights, and then returning back to civilization. But I don't think that's any different from SV/ES.
Overall I think SS is a great game that should give you what you are looking for based on your criteria.
My only dealbreaker from enjoying SS regularly is that performance is trash. And to be fair I think that has everything to do with being built in Java. I've got a fairly modern PC, not a gaming PC, but it can run modern games like Star Valor, Endless Sky, M&B Bannerlord, Kenshi, etc. with good performance and for days if I wanted, and not be a problem. But SS leaks memory like a sieve. I'm lucky if I can get 2 hours of gameplay at a time before performance grinds to a halt and I've got to kill the application and relaunch. And that's if I get the option of saving and killing it myself, more often than not I get a flickering screen and then the "game" progresses on while I am just left watching a flickering still image from a battle that happened 20 minutes ago, and I have to resign myself to the fact that whatever I've done since my last save point is abandoned. However it was designed, it does a poor job of releasing memory of game data no longer needed, and basically just gobbles it up until your computer seizes. Maybe I'm alone in that, but there are a number of forum threads of people asking how to make performance better, if upgrading to Java8 will help, how to alter computer files to increase the size of virtual memory allocated to the game to improve performance, etc. I'm left baffled as to how this game with relatively outdated 2D graphics fairs so poorly in performance versus newer titles like Bannerlord with 3D graphics and 100x more AI agents making decisions in a battle. I chalk it up to a smaller dev team and a longer development window built on more archaic code/system, and still applaud the dev effort, but lament that the performance issues massively impacts my enjoyment of what would otherwise be my go-to game.
