What's the construction method of the building ? I'll bet the clicks and high frequencies of the slap even though are loud in your room, are high enough frequency that the floor/ceiling isolation is good.
Isolating the endpin is problematic because in order to get isolation, the resonant frequency of the (bass + plate it's standing on) along with the "spring" - a pillow or foam , say - has to be well below the frequencies you need to block. That means you need a very squishy spring if you need mechanical isolation . In my tests above, we tried a plate of 1/2" plywood on a ~2" stadium seat-cushion , which was about as wiggly as you'd want to hold a bass on , but it only provided minimal isolation improvements in rooms below.
I'd look into clamping steel on the Ultra mute.
In very olden days, I put three smaller C-clamps on the bridge directly. A good deal more effective than the ultra mute (no measurements) but really chewed up the top of the bridge.