I have been working on a sci-fi story for a little while now, and have recently discovered that I made a very big error in assuming the position of Eris. The main issue is that I had forgotten to take into account the fact that Eris's orbit is radically askew from the solar system's plane, 44 degree inclination askew. I had figured it would be slightly off like Pluto, but calculating its position in the year 2428 puts it at roughly 84 AU from the sun and about as far away from the rest of the solar system as it gets. It would be almost perfectly at the bottom of the image below.
With that understanding, I have to get a group of ships from Mars at 1.5 AU to Eris at 86.5 AU (in the year 2428). This puts me at an average speed of roughly 1 AU per day. This is kind of ridiculous, considering I am trying to work within a certain level of reasonableness, and had previously expected my ships to meet an average speed of 1 million kph, not 150 million kph. What I really need is a constant acceleration to the midpoint, and then deceleration to destination.
What would the constant acceleration be given those perameters?
Also, might it be more efficient and/or economical to slingshot off a gas giant after leaving Mars instead? If so, which might be the most reasonable?