Oakesje, welcome to the forum. Is the reach to the handlebars ok?

Now that I have seen the bike, I think I would cut out the top tube and the seat stays and replace with new parts at the desired standover height. I don't think you can move the top tube down much at the head tube, maybe a little, so the top tube would be slanted. Modify the seat tube as necessary. If it were me, I would sleeve the seat tube to reinforce the joint at the seat stays and top tube.

I would not splice a tube in the middle. Just cut the old one out and get a new one in there. I don't think we understand the loading of this bike well enough to be possibly compromising fatigue strength