For this mission, you don't need to slow down or stop. Just keep taking image data starting at 548 AU and keep going until you're at 900 AU.

I thought you have to move laterally to get the pixels? "The data are acquired pixel-by-pixel while moving an imaging spacecraft within the image."

You can already do that while slowing down. Might require some image correction though to account for that in the pictures.

With a solar sail you can just start tacking, right?

I think a solar sail mostly works at a broad reach or a run, so it would be more of a jibe than a tack ;)

I don't know anything about sailing but doesn't tacking depend on some sort of keel?

With a solar sail you can "tack" by reflecting photons against your tangential velocity – but that only works if the tangential component is indeed what you want to shed. Getting rid of radial velocity is more difficult, and in a hyperbolic (escape) orbit radial is mostly what you have.

Probably not at 540 AU. Not many photons out there for the solar sail to grip.

Can't you use the same lens to focus the propulsion laser?