If the picture shows the space you want to fill with a fully grown plant, then right now is a fine time to flip to 12/12.
When an area is roughly half-filled, it's time.
Yes, 38 liters is a big pot. you need about 4L / month and that errs on slightly large size.
is it slowing down, or is growth more distributed? It's tougher to ssee when it is spread out vs top colas racing an inch or more per day upward. Did they get taller and you didn't raise the light to match? This can slow down vertical growth, if so. Top leaves look light green, but more than one thing can be the cause of that than just some minimal cannibalization from slightly too much light. if related to nutes, that could slow growth a bit too.
