Well, I haven’t really thought it out by now and it might well be a bad idea.
Anyways I would still keep Exegol around as the planet with the Sith fleet. But the Way-finder would lead Rey to Ach-To. Maybe Kylo didn’t visit Ach-To, because the Holocron needed to be opened and he couldn’t or was lead to an other place (Sith-fleet Exegol, maybe?) or maybe he just did not bother with the Sith (let the past die, etc.). My favorite idea to answer why he does not use the Way-finder is that he is afraid of what he might find. The oracle-scene (if it exists?) would really help to sell this point.

But anyways you are most certainly right that the movie would be rather short.