They pretty clearly spell out that Jabba has Han and that they will be meeting at a distant point in the future at the rendezvous but to have planned it for a full year away seems a bit unplausable. All I can think and read into it myself is that it took a lot longer to get everything in place that they planned
For Lando to create a character, gain trust in Jabba’s crew and become “one of the boys” had to take same time. We’re talking about the galaxy’s greatest gansta’ leading the galaxy’s greatest gang. You don’t just walk into that place, buy the room a drink and get accepted.
Luke has clearly advanced in skills. There’s certainly some time tied up there.
The entire Rebellion has evaded the Empire, regrouped and found a temporary place to hide while also fining out about the new Death Star, examining the layout and finding a way to destroy it.
A year seems easily plausible to me.
I get the feeling you think I’m of the opinion that ROTJ comes nearly straight after TESB, where in fact I grew up always feeling that a not insignificant amount of time (such as a year or more) has passed between the two because of what I listed further above (you might want to check a few posts back where I basically cover what you said about Luke in greater detail, so that point is moot in regards to responding to my view on time elapsed) and many other things that just gives you a general feeling of time having passed without having to try and specifically quantify it.
It’s only when getting into discussion about it that I’ve then tried to nail down a time period for myself based on how I interpret the movies and other information I have since learned (or been reminded of) and it’s because of what Luke says to Lando at the end of TESB that alone makes me now say a year sounds implausible as to what they had planned themselves then and there but obviously other factors have blown out the rendezvous date from what they probably meant to only be 2-3 months from when they parted ways. Which in the end is probably a good thing otherwise theoretically Luke wouldn’t have been the same Jedi Knight we see in ROTJ and probably couldn’t have pulled off the rescue.
So I don’t dispute any of what you just said, however I don’t think it would take a full year for Lando to infiltrate Jabba’s palace. We’re not to know as the theatrical audience how big Jabba is within the lore so I don’t think the “galaxy’s greatest gansta’ leading the galaxy’s greatest gang” has much stake in the plausibility of Lando’s infiltration.
There is also an issue in the line of thinking of actually being indoctrinated as one of the gang as then at some point Lando would have had to carry out acts that proves his worth to his employer and colleagues which likely then makes him not really suitable anymore as an honorable and trust worthy member of the rebellion (he’d probably have blood on his hands). The fact he is hiding his identity behind his face guard to me confirms that it is an actual infiltration that runs the risk of being found out at any time through recognition that he isn’t actually one of them and this is why I don’t buy that its part of the reason it has taken upwards of a year to conduct the actual rescue itself.