“Flip and burn at the halfway point.”
Even a whisper of gravity is better than nothing.
People don’t or can’t do the math needed to grasp what even small constant acceleration can do.
Ignoring the relativity limits due to the apparent increase in mass as a body approaches light speed. If you could do 1G acceleration in the spacecraft the occupants would stand and move around like on earth in less than a years time they would be approaching 99.999 of C or light speed. From earth they would have appeared to travel half a light years distance and would be receding at apparent light speed. Time dilation for the crew means they could cross the galaxy in what would appear to them to be a lifetime but would be 100,000 years to an outside observer. It’s all relative my boys. A single grain of dust at 99% would vaporize the ship so some form of energy shield would have to plow the road up front even the drag of the interstellar medium primarily hydrogen gas would produce a huge shock way up front at thuse speeds.
Point is if you can have even 1/10 of a G of constant acceleration you can get to truly fantastic speeds in a few weeks or months time. Sending a probe to 100AU and the gravity lens point is doable in the career span of the scientists launching it. Mars in a two weeks comes into play and orbital windows don’t matter you can cut across the solar system directly no transfer orbits direct hyperbolic paths. Jupiter and Europa also direct and slow into orbit or landing on Europa.
High energy as in gigawatt level plasma thrusters allow for hyperbolic paths and deep space exit velocities that get probes to alpha centari in hundreds not thousands of years, using a magsail to slow down and get into orbital capture of the Centari system is possible too. 30% of C is within reach with multi megawatt plasma thrusters for probes. Drop down close to the Sun via reverse orbital thrust vector down to near Mercury orbit and then turn and burn deep and hard in the Sun’s gravity well using a thousand plus Sun’s worth of concentrated solar at gigawatt levels that close to good ole El Sol your exit path is a straight line out at 3 to 5 Gs of continuous acceleration you should hit 30%C as you scream past earth’s orbit