In the Expanse, ships using the Epstein drive are often capable of continuous high thrust for months at a time. Thus many ships routinely don't plan paths that are "optimal" in terms of energy consumption. Rather they intend to be continuously thrusting at a decent fraction of 1g, to provide gravity for crew comfort. Thus they plan a trajectory that will allow them to reach their destination while accelerating the entire time (changing the direction of acceleration at some point so as to arrive at the destination at the desired speed as well as the desired position).

Such a trajectory is definitely not a straight line. It's a complex curved trajectory influenced by the Sun's gravity (and that of any planets you pass close enough to) as well as the acceleration from the ship's thrust over the course of the journey. But it's definitely much "straighter" than just coasting on an orbit.

On the scale of the entire solar system, the Sun is incredibly tiny. The odds of any desired "straightish" path needing to pass dangerously close to the Sun are actually pretty low, even for destinations that are on the other side of the solar system. And even if the shortest path would pass too close to the Sun, you don't really have to "go around it" in the sense of taking a large detour; just angle your initial path a handful of degrees off the direction you would have taken, and now you'll miss the sun by millions of kilometres. It probably makes the course calculations more complicated than just "thrust in one direction for half of the travel time, then turn around and thrust in the exact opposite direction for the remaining time", but that can be worked out. This deviation would turn a "straightish" line into a somewhat less straight line, and thus add a relatively small percentage to the travel time; not into a massive semi-circle that takes significantly longer to traverse. (Doing even a distant flyby of the Sun like this means countering all of the orbital energy of the object you departed from, and then regaining a comparable quantity of orbital energy at the other end, which is pretty massive amounts of energy; so maybe a more curved path going nowhere near the Sun is what you would get with up to 1g of acceleration anyway, and the need to detour would never come up)

So the fact that the trajectory is a little longer than "optimal" by keeping clear of a margin around the Sun just isn't that narratively significant. The fact that it's much longer because the planets are very far out of alignment might be more significant, but in general the Expanse books don't actually try to sketch out the actual planetary alignments (so far at least; I'm about halfway through book 3). They do mention journeys taking weeks or months, so I just always assumed that some of those journeys between points in the Belt were "cutting across" the inner system to a greater or lesser degree, and that was the reason they were taking many weeks.

It's also worth noting that most of the navigation (in the books I've read so far) takes place in the asteroid belt, or the outer planets of the solar system (at least as far out as the Belt). Orbits in the asteroid belt take multiple years, so if two objects are relatively close together it's going to take many years before they're on the opposite side of the solar system (and vice versa). So over the time scale of a single book, these distances just form the geography that navigators are working with; they've been vaguely similar for a long time, and will stay vaguely similar for a long time, so the specifics wouldn't be hugely noteworthy.

As you go further out the orbits get even longer. The ring is said to be beyond Uranus; Uranus' orbit takes 84 years. The ring is essentially going to be "on the same side of the system" for decades.

At the start of Abbadon's Gate Holden was trying to take a job that would send him to the opposite side of the solar system from the gate, so the books do mention the concept of being on the far side of the system from some other destination. But generally it's the travel time the books talk about, not the orbital positions that cause the specific travel times.