I have seen several posts indicating that once you reach a certain distance from the centre of the galaxy (Sagittarius A*, a supermassive black hole), that route planning stops working, and all further jumps to reach the centre must from that point be plotted manually.
On my approach, at about 2000LY, I did indeed find that the route planner stopped dead when trying to plot a just under 1000LY route, and could only manage around 250LY before giving up entirely.
Not being one to walk away from a challenge, I have found a solution:-

All that is required is to switch the view in the galaxy map to realistic. It still takes a bit longer than normal to plot a route, be here is the proof:-


Sit back and enjoy the ride, home in time for cornflakes.
On my approach, at about 2000LY, I did indeed find that the route planner stopped dead when trying to plot a just under 1000LY route, and could only manage around 250LY before giving up entirely.
Not being one to walk away from a challenge, I have found a solution:-

All that is required is to switch the view in the galaxy map to realistic. It still takes a bit longer than normal to plot a route, be here is the proof:-


Sit back and enjoy the ride, home in time for cornflakes.