It's too late to prune them now to improve their performance this year, but you have a few options for next year.

Hydrangea paniculata often do this flop. It's typically a result of pruning technique. Some people like the big flowerheads so they remove last year's growth every spring (basically cut it back almost to the trunk). The plant puts out a bunch of new, weak growth, that bends under the weight of the huge flowers. 

You have a couple options. Right now, you can cut off the flowers and put them in a vase to enjoy. That will take the weight off the branches and allow them to spring up again. You can take off 12=18 inches for a stem. When you make the cut, make sure you cut about 1/4 inch above a node/a set of leaves (these are called heading cuts).

Or you can wait until next spring to prune. This is what is recommended by the company that sells 'Vanilla Strawberry' and it allows you to enjoy the dried flowers over the winter. Once the weather starts to warm (late April-May), you will start to see buds swell along the branches. Cut back the newer growth (growth from summer 2024) to right above a bud. The plant is going to push new growth from this bud, and it's often a few feet of growth over the season, so prune with the future growth in mind. The plant will start to grow. After it has put out, say, 6-12 inches, pinch off the tips of all the new growth. This will force the plant to branch again. You will end up with more branches and more flowers, but the branches should be stronger and flowers will be smaller. Therefore, less flopping. 

I would also refrain from fertilizing the plants with a high nitrogen fertilizer because it can cause them to put out weak growth that tends to bend and flop. I wouldn't recommend fertilizing these hydrangeas at all, unless they start to show deficiency (yellowing leaves or another sign). 

I mentioned heading cuts. Here is a link to a video from Minnesota that shows pruning techniques on hydrangeas. At the 2:50 mark, it explains heading cuts on a hydrangea like yours.