Apparently you are not the pro in term of limited restrooms stall. You need to “floor jumping”: the art of using the stair either up or down a few levels until a free one is avail.
We did that! Sometimes I would sprint across the stairs 1-2 floors to go to the bathroom. The problem was that just to reach another stall you had to go through x4 security doors that were quite hard to open so it took quite a bit of time. Something that should have taken max 5m now was taking 15m, and my bladder was quite unhappy. Also, due to this, all breaks got elongated, as we were waiting for colleagues before going to lunch (for example) to go to the bathroom, and we were also waiting during meetings for them to arrive from the bathroom.
Then you might have the same problem there so you end up "toilet hunting" (if you're lucky and your company has multiple floors) which is mind bogglingly time inefficient.