I had a similar problem on my sat-for-a-long-time 76. I used to turn the key and hold pressure until it moved. Sometimes I gave up. One of my keys is still twisted. Doesn't sound like a key problem since the once the cylinder moves the tumblers are already fit to the key.
One clue to me that it was a sliding surface problem inside the door poarts, helped by lube, was that it was worse when it was cold. I think that I adjusted the length of one of the rods also, the threaded plastic piece. Don't know how it got out of adjustment but moving it helped.
Can't get to anything without removing the panel, unless you take out the striker plate, I think. Even the door handle and lock cylinder are attached from the inside. Then you can just hose lube in and hope you hit something. You could probably just take one screw out and bend a WD-40 tube. And hope.
I spent a lot of time inside my doors but eventually got them working right.