I've had a look back on my photos as in my last house the neighbour had one which was years old. The stems look the same as yours and they have put a support behind it. It will never have the trunk of a tree as far as I https://www.kelways.co.uk/page/tree-peony-care/10/
This tells you how to prune it.

I would wait until it goes dormant in winter and then cut it back down to the ground. It will grow again next year with leaves from the base. It will not flower next year as the flower buds are in the stems that you have cut off but will start flowering the year after. We have an ancient tree peony which I try to control this way, the trouble is that it grows back even stronger than it was before!

Thank you Cammomile and Bulbaholic for your comments. I will go down the route that you suggest Bulbaholic and cut it back hard when it is dormant.