Better layout of copper planes
Since the DC/DC converter is isolated, it is important to maintain this isolation. To this end, power and ground planes (as well as individual PCB power tracks) should not be placed too close to each other, to avoid parasitic coupling.
However, this might require revisiting and rethinking of the original concept, since currently a certain level of proximity is required due to the fact that we have power (barrel) connectors on both ends (left and right) of the board. Perhaps a 90 degrees rotation of the DC/DC converter (around the 'Z' axis, perpendicular to the board) could lead to better separation of the power planes.