I second this.European mountain ash (Sorbus aucuparia) is my guess.
I second this.
I echo this.Looks like a native Mtn Ash. Better as a landscape tree rather than a bonsai.
Poison sumac.
Even if they won't reduce, one can always cut a leaf to 2 leaflets or 4 or 6 or .... Petiole length may then be a problem, but so what? One point of deciduous bonsai is the beauty of the 'skeleton' during the winter --> internode length. This was the coup de gras of my past toying with mountain ash.Probably for larger trees due to the size of their compound leaves. But they can be reduced.
Kusamono if it is young and larger bonsai from bigger collected or ground grown trees.