Fence post leaning from poorly drained soil

I put up a fence last year.I dug the holes, put down gravel and used concrete, however because of the soil, I could only get down a max of 3' is most spots, averaging around 2.5' down per hole. I realize that 4' is best for the frost line here but after that 2.5' to 3' mark I was hitting rock hard clay that a jack hammer and power auger wasn't even touching.

This part of my lawn does have relatively poor drainage. It's sloped but the combination of my neighbours yards sloping into to creates a fairly wet corner where 3 of my posts sit. How can I fix this so my posts will no longer lean? I am open to digging it back up but not sure I can get down far enough with the clay to make a difference.

My 2 thoughts were to dig it up and try and taper the hole out like a big foot for a deck would, or potentially take some long rebar and drive that down into the clay a couple of extra feet (if possible) and reset the post in concrete with the rebar. I am completely open to any ideas because I do not want my new fence to keep leaning and I have to fix it year after year