So, if you're unaware, the basic math of PF2e's armour items assumes that for light and medium armour, (item bonus + dex cap) = 5, and for heavy armour, = 6. This means that as long as you can pick an armour that has a lower dex cap than your dex bonus, you are expected to be getting either 5 or 6 AC from your armour.
With all that said, there are a ton of armours out there that offer some kind of tradeoff, where that formula doesn't hold. For example, the Armoured Coat gives +4 AC between item bonus and dex cap, but it has the flexible trait which negates the check penalty to athletics and acrobatics checks. In my mind, that's a crazy tradeoff to make. The extra AC is so much more important.
At the end of the day though, I don't have the time to analyze the tradeoff offered by all of these "sub-optimal" armours. So yeah, I'd love to hear if anyone has experience with an armour item that yields less than 5 AC (or 6 for heavy armour) and yet the tradeoff was actually worth it for them.
Breastplate is good even when you can't max its dex cap. Sentinel is only 1 lv 2 feat, after all. Not really what you mean, but I've never considered anything lighter.
Upon further thought, I'd say that any "situational" armors like armored coats are simply not worth it since armor runes take up so much of your budget already. Having a backup for niche situations is just not economically feasible, and rolling with worse armor all the time just invites crits your way.