Joi is the only thing in the entire movie which represented a personal decision on K's part. That is why she is important. He chose to buy her, he chose to upgrade her to bring her with him.
You can't justify her being needed for the plot by referring to things K did for her solely so as to support her presence in the plot so that she can carry her task of adding sex appeal, being a love interest, and verbalizing K's own feelings. If she is not there, the remaining plot still fills all of character development arc.
His connection to Deckard is through a false memory implanted into him. His connection to the rebels is through a case assigned to him.
So? Rachel had implanted memories, and Deckard may have had fake ones too, we don't know, that didn't prevent Deckard from protecting her, in fact he developed empathy for her as a result of it all. His own connection to her or in the end to Batty was also merely through the case assigned to him. It's what he experienced that changed him, not figuring out what was real or not, or who told him to do what.
Everything you can try to use to justify K needing Joi, Deckard experienced as well through his emotional connection to characters that are actually in the plot for actual reasons in BR1. Plus he may be a Replicant too, or not, it doesn't matter, the result is the same.
Like Deckard, like Batty, like Rachel, the memories being fake or not are irrelevant, it's what he has
seen, experienced, that makes him who he is, not someone telling him (or us really) what he is or what he is feeling or should feel.
All of this applies to K without Joi's presence, who actually contributes nothing to any of the above that the rest of the plot doesn't already. The difference is she is there to explain this transformation. Something unnecessary, just as it was not needed in the first movie. The interactions between the characters that were actually relevant to the plot were enough to accomplish all of this.
His job and existence is something he was created for. Joi is all he has to anchor him to a sense of a self-determination. It is also what ultimately drives him to believe that what he does at the end is a consequence of his own decisions rather than another part of a fate he has no control over.
You can say the same without having Joi-Spell-Things-Out in there.
"His job and existence is something he was created for. What he experienced, false or otherwise, is all he has to anchor him to a sense of a self-determination. It is also what ultimately drives him to believe that what he does at the end is a consequence of his own decisions rather than another part of a fate he has no control over since even if he knows it was false."
Joi is not needed to justify any of this. Note how in BR1 Rachel never forces Deckard to explain how he feels. It was not needed here either. Yet the arcs are highly similar. Same with Batty.