I love that there are so many ways to do something, not having to follow absolute rules and being able to play with different methods to achieve what you have in mind.
In this case, smile styles, first I tried with the one I learned thanks to @picapauyan (site here), which is the “backstitch” and is the one I generally use the most, but in this case it didn’t work because I needed the smile to start in one round and end in another.
Then I tried the most used one, which is to make a stitch in the centre of the smile with the same thread to keep it in place, but I was not convinced, the smile was too long for that method, and also the stitch was too noticeable.
And than it occurred to me to do that stitch (and several more stitches along the smile) but with sewing thread and…happiness! It was perfect.
So smile ready and move on to another part!