I think they should just add active time lore or whatever it was called from FFXVI. It's basically an ingame wiki so you could easily see a brief backstory on each character you come across. If you hadn't done the content yet, the active time lore could have spoiler tags indicating what content you would need to do to remove the spoiler tag, but you could also choose to reveal the backstory anyway.

That would help people who have poor memories, probably serve as a great resource to the devs themselves, and it would help people who don't have the time or inclination to do all the side content.

We can't go on like this with characters locked behind side content. It's going to make the side content irrelevant to the main story, which just doesn't work.