I actually think all the ones you posted screenshots of look fine.
Blindly stripping pin excerpts at the first para would literally break the excerpt of every single one of the pinned topics you just screenshotted.. hard to see why you’re even proposing that.
Have you looked at GMail? It also concats in a similar way.
concat from
Notice that Gmail does NOT stop at the first paragraph, and neither should we..
Maybe the thing to do is add some other kind of markup people can use, who want finer control than this. Certainly not a V1 thing.