I do not think Ronaldo will stop playing at the end of his contract. He likes this game very well and with the way he plays, it is obvious that he still wishes that he was young. Ronaldo will not retire at 40, he would like to play for as long as his body can permit. So, what I feel is that he will have a contract renewal at the end of this one, and it may be even better because of what has been offered to other European players that have moved to Saudi Arabia's league. Al Nassr will try to beat it, to keep him happy.
it is very possible, we can see his performance so far. Ronaldo is still able to play up to 90 minutes of normal time. Ibrahimovic is another player who was active until the age of 40, Ronaldo could be next on this list. On the other hand, his presence also greatly affects the image of the SPL, so I think he will stay and play there until the end of his contract.
For a new contract extension, I doubt it a little. Because he has reached more than 40 years of age, injuries will usually become close friends at that age.