I might believe I can still have sex standing up but my wife doesn't believe me enough to try!
About getting engorged without a firm base ... this also happened to me, specially when using a pump. One thing that has helped is to strengthen the muscles at the base of the penis. Unlike the erectile tissue in the shaft of the penis, those at the base are under voluntary - not automatic - control so you can improve them with exercise. My physio challenged me to "twitch" my cock in the shower - something that I had always taken for granted since a young age.
I had to really practice, with only the smallest response at first, but after some months those muscles have gotten much stronger, and while they don't make an erection on their own, they certainly give better support from the base to make the most of whatever chubbiness or firmness I can get in my shaft. My wife also helps sometimes - stroking up the semi while I clench and hold at the base to build strength.