Okay, since no one responded, I've spent another week trying to figure out what's going on. Here's what I've found. (Playing 2.2.5 now.)
The debris problem is still there. Every once in a while I'll fight another empire that drops debris, but most of the time nothing, even if I can see their ships have technologies I haven't unlocked. I thought it might be an issue where the AI isn't upgrading some of his ships (to where most of his ships would even HAVE the techs I want to salvage), but even in cases where I can see his fleets have several techs mine don't, it usually won't drop anything. It doesn't help that the AI empires have pretty bad technology (in 2300, none of them have reactors above tier-3, and most are still using tier-1 weapons and shields); I had to really go out of my way to make sure the AI COULD have higher tech than me.
And some non-debris bugs that are exacerbating it:
> The bugs people have talked about with 2.2.5: every planet having three Precinct Houses, almost every starbase being shipyards and nothing else. The AI has a terrible economy, as a result.
> One of my starting neighbors was a Fanatic Purifier, so he should have been really focusing on military. I used a console command to kill his starting Corvettes... and he never rebuilt them. Ever. Twenty years later another Pacifist neighbor rolled over him with ease, since he had NO FLEET. Note that I'd set my AIs to "Aggressive".
I then repeated the test with a different Fanatic Purifier in another test game. Same result.
Now, fast forward 100 years. I've conquered the entire galaxy except for the FEs, one dinky mostly conquered empire, and one single Devouring Swarm empire I'd left alone. (There are also three or four vassals I have yet to integrate, but they don't count.) I had ~220 inhabited worlds, the swarm had 15, with 511 population. He'd conquered three AI empires previously, and settled the worlds that fit his habitability. I used a console command to switch to controlling his empire, and there are other things I've found:
> AI empires are building starbases around every potentially habitable planet (even if it's not a type THEY can inhabit) and every wormhole or gateway. Even if this takes them well above their starbase limit. This empire was at 25/20; sure, some of that could be leftovers from empires previously defeated (since DSs can't assimilate enemies), but it's still a problem. And yes, every one of those 25 was a 2-shipyard base (with one crew quarters and one empty slot), except for the capital's station that had the one trading post.
> The AI hasn't upgraded any of his starbases, even though he has the technology and has ~7k alloy to spare.
> He has no money, and an income of only 18 energy per month, but has maxed minerals AND food he could be selling on the market.
> Likewise, the AI has two fleets, of power 13k and 15k. Both are in need of major upgrades; most of the AI's destroyers and cruisers are still using tier-1 components, even though he's now unlocked tier 3 reactors and such. The upgrade costs are ~4k and ~8k, respectively. New constructions will use the best technology, but he never upgrades the older stuff, so his fleets are ridiculously slow.
> Despite having max minerals (19k for him), every planet has at least three unused building slots. Except for the most recent acquisitions, of course.
> Two of the three science fields have no scientist appointed, which really cripples research. And yet, he's got six science vessels with captains that aren't doing anything (they're not even assisting research to gain XP; they're just docked). He also has six admirals, for two fleets.
So I thought, maybe it's just the Devouring Swarm that's broken, so I switched to the mostly-conquered guy. Four planets, 87 population. (He was mostly wiped out 30 years back by a Fanatic Purifier that I recently conquered for myself.)
> 8/11 starbases. Four shipyards, four bastions. Again, none upgraded even though he has the tech.
> No low-tech ships, probably because they all got destroyed earlier. Only 19 points of ships in his entire empire, despite being at war with that Devouring Swarm. Still needs a few upgrades, though.
> No missing leaders on research, but the ones that ARE assigned are idiotic. A Physics (Field Manipulation) researcher is running Engineering, while Physics is researching a Field Manipulation tech with a Maniacal leader. Two Spark of Genius leaders are on the science ships.
> Capital is full of destroyed buildings that have never been rebuilt, despite having 14 population. (Obviously not the original capital.) Other planets aren't terrible other than the bug with Precinct Houses.
> Again, max minerals and max food but no energy (637 -71).
So yeah, the AI is really borked and won't put up much of a fight in the latest versions. The debris was just the most obvious sign that something was wrong.
