I see where you’re coming from and it plays well with “the jedi of the past” thing, but looking at it from a lore and canon perspective, it doesn’t make much sense to have Jedi other than Luke, Qui Gon, Anakin, Obi-wan, Yoda and (possibly Leia) communicate from the netherworld of the force without having discovered that knowledge.
Why not? First of all, Anakin died without having learned that power, and yet he can still become a ghost. More importantly, it shouldn’t be a struggle for Rey to speak to the ghosts, no? And yet it’s something that takes practice and training. The way I interpreted it, the challenge of hearing the voices of the Jedi that came before is hearing not just the ghosts, but everyone.