By the time of ROTJ, it’s been like 3 years since Boba Fett delivered Han. (At least if we ignore supplementary material like Shadows of the Empire.)
I still find it hard to believe there’s 3 years between Empire and Jedi, did it really take that long to rescue Han? But the idea that his relationship with Jabba was strengthened between films is a plausible theory. Another being he just hasn’t been paid yet, and Jabba is making him protect him if he wants to get his payment.
I don’t have a contemporary source at hand, but my recollection of the timeline around RoTJ when it was released was that it took place months after ESB, not years. Looking at various online timelines, consensus seems to be that it’s a year after ESB, which seems to be stretching things to to fit neatly into a BBY/ABY structure, but still not 3 years.
That aside, lots of interesting ideas in this thread for fixing some of the narrative issues with RoTJ. For my part, everything wrong with RoTJ is rooted in a set of decisions, some in reaction to the experience of making ESB, some in reaction to Lucas just running out of energy, that shrank the scope and mangled the feel of the universe in order to just wrap the fracking thing up so that George could get on with his life.