Definitely not too late! In fact, this is why I recommend applying as early as possible -- around this time, you can gauge how things are going and see if you want to add on any more schools. If it's financially within your capabilities, go for it!
In terms of the schools you add, you'll just want to make sure you are strategic. If your goal for adding more schools is to get more interviews and increase your chances, I'd definitely suggest adding some programs that are lower-ranked and also looking into some of the non-MSTPs that are bigger-sized, funded, and have a good track record (I wish I had done this earlier on when I wasn't getting as many interviews as I hoped. Always good to have more invites/offers than to not have enough!). If you are doing even better than you anticipated though and you figure "why not?", then adding top-tier schools is fine! 🙂
You can also look at those schools specifically to see if interview invites have gone out yet and how soon their first interviews are. If there were a school for example that had let's say 2 out of their 4 interview dates super early and had already given out invites, I'd probably advise against applying there. But I think you are still in good shape pretty much all around and shouldn't be super late. I'd recommend looking up the secondary questions and pre-writing those, even though you'll probably get the secondaries within a few days anyway. The sooner you can submit, the better!
Good luck!
