Try a tomato cage, and tether the plant to it high on the stem when the bud starts to form.

Some of the larger "dinnerplate" variety get so big and heavy they crack the stems. Providing support will help that out.

When large blooms are opening and rain is forecast, lots of us sneak out to cut them to avoid disappointment after the rain. Some actually stake each individual flower on the big ones, but that's a bit too intense for me.

Staking issues?

Looks like you're using bamboo stakes... Try three, and teepee them together.

Too late for tomato cages this year, it looks like. There is one 'overflow' garden I have that I use the cages, but they always topple over late in the season after a rainstorm, as the ground is rocky there.

Some folks cut the stake part off the cages, flip the cage so the larger part is on the bottom, and bend the cut off stakes, pushing them into the ground to hold the cages stable. I never got around to finding the correct tool in hubby's garage to cut them off, so mine are still used the traditional tomato way, often tottery and prone to collapse. I don't worry about them much, since that's only a dozen or so plants.

There is a reason seasoned dahlia folk grow in rows.... Supporting the dahlias are so much easier when you don't have to fuss with each individual plant as much. Instead of staking each individually, put posts in all four corners, and wrap twine on different levels to support them. This year, I have horizontal fencing 18 inches up that they grow through to keep the stems straight. It's getting about time to start wrapping the rows.

Wrapping row image from last year... I am using the Horizontal fencing this year to keep the plants from falling on each other within the rows.

Individual staking at a private garden...