My DD was doing the same in the beginning and as she an amazing ability to hold on, it comes out with alot of force when she goes. She would sit upright and legs out while holding on to the sides. We don't use the toilet insert as she doesn't like it.

Most of the time she tells me when she needs to go to the toilet so I take her nickers off completely.

I gently encourage her to sink down a bit or put her legs either side of the toilet seat so it creates a down stream rather than forward, if that makes sense. She also like to watch the stream come out, so now she almost creates a lean forward trying to do that.

I think once she gets to CC and sees how other girls go, she will follow suit. It will click eventually.

We have also had the situation of when she wears longer dresses, she has trouble holding the dress up and holding on to the sides and the dress gets wet.

I TT two boys prior to DD, seemed so much simpler