have you checked the number on the transistor to see it matches the others?

have you checked the kicker return spring to see it's not maybe been changed with a thicker spring?

is the plastic L shaped kick out arm free to move, like not touching the playfield cut out?

maybe polish the zinc plating on the plunger so it's real smooth?

is the raised part of the wireform of the switch that comes up from the underside possibly sitting too high and the ball is losing momentum from not easily closing the switch?

have you checked the rear of boards for any dry solder joins?

personally if your going to repin J4 then repin J1, J2 & J3.

it is possible the previous owner has severed some strands of wire by pushing on the IDC's and in effect reduced the current and increased the resistance causing your weak kickout issue.

if any IDC (Insulation Displacement Connector) looks suspect I suggest replacing them all with crimp type as you have considered.