I think ‘The Last Jedi’ refers to Luke abandoning the conceptual ideals of the Jedi, which is what led to their downfall in the first place (and possibly what led to his failure with Ben and the loss of his students).
Luke will be the Last Jedi because he abandons those ideals when he trains Rey. Rey’s training will embrace both the dark and the light, but only in an attempt to do what is right and just. We saw him sort of embrace that in ROTJ, and maybe that scared him, but after losing all of his students and reexamineing the ideals of past Jedi, he realizes that suppressing the dark side isn’t the best way to go. So he teaches Rey to embrace love, and to accept possesion and attachment as a natural part of life and the force.
So he, in the end, truly is the last Jedi. That would be a good way to both acknowledge the prequels, but also distance itself from them.
But I do think he will at least make it to Episode IX
This does not make any sense. If you abandon the conceptual ideals of the Jedi then you are not Jedi any more.