Ford suggested to Lucas to kill Han in ROTJ because he thought the last act missed something like that, not because he wanted out (he already knew that Lucas had no immediate plans for a fourth movie, the decision was made even before filming ROTJ). Lucas rejected Ford's suggestion because he wanted an happy end, and above all because he thought that killing Luke, Leia or Han, all beloved characters the audience identified with, would completely steal the show from the redemption and death of Anakin.
It's not Ford who was hard to convince to reprise his role, it was Hamill who secretely hoped that Ford would refuse and that would be his excuse to refuse as well. When Ford rapidly agreed, Hamill decided he couldn't possibly refuse.
When Disney approached Ford, Solo wasn't supposed to die. He rather ended up reunited with Leia. The decision to kill Han was a late one, during rewrites of the script. Abrams and Kasdan realized it was unavoidable if they wanted what they had in mind for Kylo to work. They needed something very strong that would mark without a doubt that he had totally fallen into darkness, something that would make people understand there was no way back to the light for him. That's when they told Ford they wanted to kill Han in the first movie, and he had no problem with that.