I think you'll be happy to know that Chara will still be in the story in a way! It's a little roundabout but they are still very important. The mysteries of Frisks many sins will be a little side plot that will be increasingly important if you spend more time around Sans since he's specifically suspicious. He doesn't remember the resets but he has theories about them. He is very distrustful of Frisk because of this and will be a bit cold to MC too, although the shock of Frisk having a grown kid that they never told most people about will probably shock some of the cold out of him lol.