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D. oelandica fits better, but I wonder if it's big enough for oelandica (15-20 mm, the biggest of the Dioctria covered in my books of Netherlands, Belgium & Germany):
Wolff, Gebel, Geller-Grimm (2018): Die Raubfliegen Deutschlands
van den Broek, Schulten (2017):Fieldguide to the Robberflies of the Netherlands and Belgium
D. oelandica is a species of deciduous forests. Did you compare the female D. cothurnata