I don’t buy the flying saucer stuff, it strikes me as at best creative speculation.

I'm glad we're in agreement there.

For example if you were viewing technological development on different planets, all other things being equal but one has a fractured global government with hundreds of different defense concerns and intra-class conflicts sapping resources and scientific ability, vs a planet that has progressed beyond that, and is able to direct all resources towards a single purpose. One of those is going to be more effective in material terms.

I also agree here, though I think the importance of the speed of progress isn't the important part here. Even if it takes a billion years, it potentially wouldn't matter because an extraterrestrial race could have evolved a billion years earlier than us. Though this gets into the problem of stability over long periods like you've mentioned.

I would actually point out that we have an extremely pressing example on our current planet where the endless-growth orientation of our economic system is de-terraforming our planet before we have demonstrated any ability to re-terraform it, when you’re looking at things on an interstellar timescale this starts to look like something self-defeating rather than a ticket to rapid development.

Absolutely, but another thing to consider is that it may not be a requirement to have a fully habitable planet. Earth has already, since before industrialization, had places that are effectively uninhabitable to humans. We're reducing the area that is habitable at a terrifying rate, but it could be the case that it becomes irrelevant. My mind goes to a world like that of Earth within the Warhammer 40k series. The Earth is just fucked, plastered and cemented over, with a poisonous atmosphere, etc. They just brute force the problem by ignoring it.

That, and living on a planet may not be necessary in the first place. Given the composition of our asteroid belt, it has basically everything we'd ever need for potentially thousands of years, easily available and minable, relatively speaking. Outside of food problems, it could potentially be possible to live nomadically in space.

any systems which are not at a stable equilibrium will simply not exist long enough to actually have an impact.

Maybe, maybe not. It could be the case that wormholes, alcubierre drives, and other forms of FTL shenanigans are impossible, and the only way to get somehwere else is to send a generational ship. If that's how things play out, intelligent life might hope from one system to the next in a manner similar to conway's game of life. It isn't at equilibrium, because each planet is drained of resources rapidly, but there is enough momentum to keep things going.

The rationale is essentially just that any system stable enough to actually sustain existence at those timescales across those distances would necessarily look different, and could not look like a system with constant boom and bust cycles, as eventually the technology gets the the point where the ‘bust’ is a self-annihilation.

That could be the case. It also could be the case that those timescales aren't needed, because FTL is somehow possible. We literally just don't know right now.

To add even more to this complete speculation, it may be the case that there are stable political systems, without boom and bust cycles, that aren't capitalist, socialist, or communist. It could be the case that technology inevitably advances such that the bust is self-annihilation, but it could also be the case that technology inevitably advances such that there isn't any possible way for their to be a bust, that it isn't possible to revolt against the bourgeoisie/monarch/dictator. AI systems are going to get a lot more crazy over these next few years, and a large chunk of it will be used to quell protest and dissidents. They can already track people based on the gait of their walk, what when (not if) that technology is expanded upon? The technology is very clearly here to use small scale drones to attack individual people, what happens when (again not if) that technology is expanded upon? We may find ourselves in a position with the bourgeoisie impossible to touch thanks to technology.

This question of economic/political stability is essentially just one possible answer to the fermi paradox and great filter.

I guess I’m just not that confident that we’re on a trajectory which would result in us becoming an interstellar civilization without the need for major overhauls to our political economy first,

Yeah, I agree.

and it makes it hard to envision aliens getting to that point while still also being tied up with internal ethnic strife and economic crises.

Maybe I've just focused on science fiction dystopias too much, but I can envision it. But that's not to say I think it likely.