I don’t think there’s a way to keep the muscles tense subconsciously

Part of 'better posture' is relying on your core muscles for stability instead of your back muscles, and that's the thing that keeps fucking with me. I overly rely on my back to keep me upright, and I have a hard time keeping my core engaged like that too. From what I've been told, it's a practice thing and I just don't do it enough for them to stay engaged without thinking about it.

The office chair is providing something called 'lumbar support' if you want a search term, and some people do have a bigger curve there than others. I'm proportionally short, so I have to move the cushion around until it hits the sweet spot and isn't uncomfortable. Slouching will also make your lumbar curve flatten out, so that could be it. I think it's just a matter of trying different things and seeing what works best for you, it's not one-size-fits-all.