I have a brush that is far too floppy. It is ~19mm but set at 60mm. Not only do I like the handle, but I love how soft and comfortable this vintage brush is and would love to work out a solution.
You could sort of pinch the brush with your fingers when you hold it, it would change the effective loft and give the brush the illusion of more backbone.
You could always try steaming the knot out of the handle and drilling 10-12mm deeper and reset. Sent from my iPhone using Tapatalk
All of the above suggestions make sense. The only other thing you can do is to modify how you use the brush. Don't press and splay the brush so much and focus more on using the tips and being more gentle with it as you lather your face. I've had to do this with a new brush that I have found to be floppier than my other brushes. Admittedly it is a boar brush which has some decent backbone even when fully hydrated so I still get enough scrub from the tips even though I am not as vigorous as I am with my other brushes. Good luck.