Yellowing Lemon Tree

When I moved in, this lemon tree was planted in a shady garden bed and wasn’t doing very well. I moved it to its current spot (full sun) about three years ago and it slowly died off in places and turned yellow in others. It has produced fruit every year since I have moved it. I can see a thick green shoot coming from the base of the tree. I am wondering what I can do to help it thrive, should I move it again? I live in north east Victoria and the seasons have been weird.