You want 8 strength and 8 charisma before you meet with Siri, ideally. Try to avoid meeting him before you have these stats. You probably also want to avoid the fourth meeting (which only happens in class, so should be easy enough to avoid) until you have 10 charisma (8 intelligence as well if you want, but that's not really necessary because there's a charisma choice AND an intelligence choice in the same dialogue tree, and I believe you can only choose one - 10 charisma is more useful in Siri's route). Also, all of his main events happen in specific places except for the first one, so make sure to use the guide so you go to the right spots. Or figure out through trial and error where he hangs out if you don't want to use a guide lol.
For Simon, do you mean making sure you have enough of a "good student" stat? Although it does open up an option that increases favour in your first interaction, the other option doesn't decrease your favour, it just doesn't affect favour at all. There are enough other choices in Simon's route (in both his main events and in extracurricular interactions) that you can gain enough favour without choosing that dialogue option. If you mean you can't choose enough "good" dialogue choices, guide should be able to help with that! If you don't want to look at the guide, my advice is to comfort Simon and show him that you believe in him/his abilities. Oh, also most of Simon's interactions happen outside of class, so keep that in mind (although the locations aren't as specific as some of Siri's are).
Hope that helps!! Sorry if it's too long, lmk if you have any more questions or are confused about my explanation!