Where on earth did this thread come from? Just showed up randomly for me and it's already 5 pages in.
Looking at all the comments it feels like "depth" isn't quantifiable, at least not consistently, in the entire discussion. I personally can't agree with making weapons/certain content rare as that would generally add to the grind. You could be like, here is your Falcon(P) with Sabot, Harpoon, Reapers after 100 hours and it would not constitute as depth for me.
Conversely, making obtaining more powerful weapons/ships/hullmods more rewarding generally involves content. Cold hard content takes hours upon hours to make. You could end up crafting entire stories, maybe revolving around a subfaction/splinter group or just throw a bunch of tesseracts in the player's face, give them a 6.5k burst damage laser that can pierce low angle shields, and call it a day. Ironically, said rewarding content can be just as likely to speed up the game since their rewards need to be powerful in some way to be rewarding.
Starsector is a game that rewards both a player's control over their own ship and their macro play. It takes some time to find the most overpowered builds but once you're rocking SO frigate/destroyer, heavy fighter spam, Macross missile spam, or just Paragon/Astral, the game won't have a lot of difficult content left to offer. No amount of depth getting TO the endgame can prevent a player from eventually reaching it and gaining the experience necessary to make the next run a stomp. It's how game mastery works in a game that allows mastery.
Factorio, Rimworld, Starsector, heck no game can maintain progression parity when you replay it unless you fill it with a metric ton of rng. You can't control progression without limiting the game to the point of reducing depth, or otherwise rendering it pointlessly inconsistent to the whims of lady luck. Adjusting your own difficulty isn't for everyone I get that, but it's one of the few viable alternatives if you really want to play again while still utilizing everything you've learned.
Ultimately, I think when any highly replay-able game gets old and easy, you should step away from it for a while. Good thing about games like Starsector is that the modding community adds on so much more content over time, and even if it isn't "depth", one of them may turn up or get updated to eventually wow you enough to come back and realize you can still have a blast playing through the game.