But someone must of discovered it for the first time. Maybe I'm looking at this the wrong way. It's not the origin of the force I'm curious about, but the Origin of the Jedi and how they first discovered the force and how to use it.
Being in-tune with the force isn't something that gets "discovered", it's the default state for all living things. Some have a stronger natural connection that comes with a high M-count (or because of it, that relationship may not be so simple.) Where sentient life in particular seems to become disconnected from the force is precisely because of their conscious and unconscious minds. An animal has no ego, no aspiration, no jealousies, no sense of possession, so they (mostly) exist in a state of natural harmony and balance.
This is why Jedi are trained young. The older you are, the more you get in your own way. Children are more naturally open and accepting. This is why we see younglings that can already do certain things seemingly without effort; it takes a lot of effort and focus for a mature mind to get itself back into that open state; to "unlearn".
They've understandably left the origins of the Jedi Order alone for now until there's an actual story to tell.
I think the general notion though is that they started out as monks; anchorites studying the mystery of the force. Slowly, over time and generations of study and meditation they peeled back some of the mysteries, learning how to quiet their minds and reach out.
I have a lot of personal theories on what happens next, but suffice to say they probably didn't start out as the Jedi we saw in the PT, or even what we will see in the High Republic. Finding the correlation between natural powerful individuals and high M-counts probably came later; the lightsabers, the role of protectors and negotiators also probably came later, by increments, over the course of millennia.
I'm sure they'll get to it eventually, though I hope they do a better job than the EU's "Dawn of the Jedi" because that was just dire and utterly uninspired. It shouldn't be like some superhero origin story. It's a religion after all, not 'The Superfriends'.