Fortunately for me, while my EX had other issues when I owned it, I never had a problem with the functioning of the doors.
I was looking up some posts in this forum about door actuators, and actually found one of your old ones in which you said the cables have gotten rusty. With their condition, it looks like the problem will eventually return. I think the best long term solution for you is to replace the entire part.
WD40 is ok for general purpose work in the garage, and it probably worked well enough to free up some rusted moving parts in the door actuator assembly. Since WD40 is so thin, it was probably able to creep and flow through the entire cable. But at the same time, WD40 seems to evaporate in short time and it's effects don't last all that long. If you still want to try a cheaper solution and would like to use something with a bit more staying power, maybe you can try some white lithium grease instead. It does come in spray form, but I don't think it'll penetrate and flow in the cable jackets as well as straight up WD40. But if you want to try it out, apply it to all the moving metal parts you see on the door actuator assembly, and give us an update on both how well it works and how long it lasts.