Random suggestions...

I am sure you know this, but the first thing I think about is clean shorts, always, every time, no cheating, ever. Make sure they're cleaned with some very gentle detergent and dried thoroughly, in the sun if possible. Take them off immediately at the end of every ride, wear a loose skirt and if you can go commando it's even better. Do not simmer in your sweaty shorts. But I am sure you know this. (I just repeat it for the likes of my dear partners, who didn't.)

Instead of chamois butt'r I use zinc oxyde cream, the same thing as diaper rash cream. Not as slippery I think, and prevents both chafing and infection of previously chafed areas. Can be applied liberally on all external surfaces (so not inside your vagina, but otherwise anywhere you can think of ...)

Drink cranberry juice instead of your usual sports drink! I really hate cranberry juice but the two times in my life (thanks goodness) I have felt a UTI coming (after knowing what it felt like from one full-blown event) I drank about a liter of that concoction and it effectively killed all the bacteria I had in there.

In the meantime if you have them often and they are painful (aren't they always?) you can ask your doctor to write you a prescription in advance for antibiotics. Some doctors here (Canada) will do it after you've had repeated UTI and know how to recognize them.

Can anyone explain to me how a cut-out saddle would help with this problem? I might be a bit slow but I don't see the link...

Good luck!