Well, I'd get to a urologist when you return home.
Cancer can spread, and who knows what is going with that?

I think you can reduce the B12 to 1mg a day for a while, and see what happens. If you have the 5mg left, just do it twice a week.

Men do get urinary tract infections, so I'd get the urologist to test for that.

500mg of benfotiamine is alot. Some is stored in the body tho and not excreted quickly. I never recommend that high a dose.
If you start back on it, I'd stay at 150mg.

Neuropathy can affect the bladder.

But I think going to a urologist is your next decision.