Very weird question but since you're a Undertale/FNAf fan I just wanted to know what Sans would think of your whole AU and what would he do?
It depends on what exactly—I doubt he’d care, given that the premise is that Chara is literally getting tired of the idea of jumping from universe to universe (now realizing they won’t be able to escape the scope of Undertale), and has decided to try living a continuous life, which, in Sans's view, doesn't make Chara a good person, but rather a tyrant looking for a new way of having fun.
Now, if the reason Sans is in UT is literally to deal with the DR anomaly—which jumped into this other universe—and if, as implied by your question, he now understands how the AU works (including things like how saves and resets operate), he would probably try to trap Chara in a softlock.