At least... The 2 brackets I replaced you can see from below, laid on your back, with a strong light angled up and at an angle towards the top of bumper/ back of tailgate. These were one of the list bits I did on mine as they were visible with good access for a spanner/ socket. As near as I can describe them oval/ rectangular with rounded corners and they are stepped to allow for difference between metal and the plastic bumper.
These securing bolts are a few of them you can see and squirt.
Again IIRC, there is probably the 25mm you are looking at in places behind the bumper where one sensors are
I guess drilling holes beforehand if you are able, might be a goer, a small chance not taking bumper off, and if you have to, you have to. Nothing to be lost but a bit of extra time spent if it doesent work Go for it !!