You have a self seeding and rhizome problem it seems. This is a plant that will infiltrate, subjugate and then dominate the garden (sounds like another problem occurring in the world but not in the garden). I have no solution to your widespread dilemma other than hoping that a few voles take up residence in your garden to eat some of this usurper away. Perhaps other members will have more effective ideas other than mine.

Dig it out - I've got this growing in a client's garden, and every year, in spring, I'm there digging up around the clump, because in one year, it will have spread below ground up to a foot all round. And I dig it all up, leaving just a small clump each time, otherwise, you're right, it just keeps spreading everywhere. When I first took their garden on, it filled an area nearly 10 feet long by 5 feet deep, and was even coming up in the lawn. If you don't want any of it, then its a fork and spade and persistence...

We got this too. The roots spread widely. Dig, dig and dig again.

St John's Wort (Hypericum) does this too. I never did get on top of it in my last garden even though I thought I'd dug every bit out. Very pretty when in flower but a real thug of the worst kind.

or if you want to get rid of it totally then a systemic weedkiller that will kill the roots [glyphosate type] is needed. a couple of applications may be needed though.