Yeah, I am recommending wall filler, to fill the gap. You can then use acrylic frame sealant over that (once you have primed the filler).
If you used wall filler only, a hairline crack will appear where the wall meets the window. You could use regular caulk rather than acrylic but you can't clean regular caulk so easily if it gets dirty.
Acrylic isn't as flexible as silicone but it is more than adequate in this case.
BTW, you may need a very sharp chisel to remove the old traces of silicone on the plaster.
edit---- although the gap is very small, the filler will help to cover the damage to the paint caused by removing the silicone.
If you used wall filler only, a hairline crack will appear where the wall meets the window. You could use regular caulk rather than acrylic but you can't clean regular caulk so easily if it gets dirty.
Acrylic isn't as flexible as silicone but it is more than adequate in this case.
BTW, you may need a very sharp chisel to remove the old traces of silicone on the plaster.
edit---- although the gap is very small, the filler will help to cover the damage to the paint caused by removing the silicone.