> We never took pictures of exoplanets. These pictures are illustrated and fake. The only pictures we have, are spectral lines and so on.

https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets

Now you can complain about how these are unresolved, and to the extent we have maps of exoplanets, they're very low resolution reconstructions of temperature, but we do have pictures.

Propulsion technology doesn't advance that fast, assuming more or less present day stuff 10 or 20 years out is fine

They've done various Moon and/or Mars missions, but nothing to the outer solar system.

As in doing a similar grand tour? For Jupiter & Saturn, it feels especially silly since they have already had orbiter follow-up missions (Galileo & Juno for Jupiter, Cassini for Saturn).

In the cases of Uranus and Neptune, a lot of the mission proposals (including the planetary decadal survey are also for orbiters rather than flybys. Trident is something of an exception.

Insert obligatory Kerbal Space Program plug here (for the more or less realistic aspects of space flight and craft design)

I mean, I'd avoid flying it through Saturn's ring plane, the ring arcs around Neptune, or especially close to a comet, but space is in general pretty empty.

There can be (IIRC, there are a few holes in some of the ISS' panels), though that's more of "make sure you have a sufficiently tough structure that it can take the hits and still provide spare power".

Also, er, I was assuming that you'd use a nuclear reactor or something given how far from the sun Neptune is.

Not really. Like, micrometeors and other debris exist, but the craft is presumably built to deal with that. The modal probe to the outer planets just goes through the asteroid belt without much care, or may have a trajectory chosen specifically so it goes near an asteroid to observe it.

Something that comes to mind is how much Δv the craft has (and what that implies for what sort of trajectory is chosen and travel time). There are thermal and power concerns, though again this is normally dealt with in the design phase.