For the T-shirt thing, a while ago I bought 3 safety yellow tech tees at Wallys for about $8 each, sized one smaller than normal. Then, since I'm not old enough to know a seamstress, I got out the sewing machine and put two pockets on the back of each one. I wear them all the time when doing solo rides and don't want to get a jersey dirty.
My next invention is going to be the JersVerter (TM). Imagine cutting the pockets off of a jersey and sewing elastic straps to the sides with clips on the end of the straps, like sideways suspenders. Then you could take any T-shirt and clip the JersVerter (Pat. Pend.) on the back, giving you pockets and stretching the shirt tight (no more parachute effect). When you get to the bar/store/dance club/organic farmers market, you can unclip the JersVerter (Copyright 2019) and you are wearing a normal (sweaty) T-shirt. Or you can move the clips on one side around to the front and have a fashion forward fanny pack. Although when you wear it on the front, I don't know if you call it a fanny pack.
Can't help with the cargo shorts, I thought we as a species had agreed to stop wearing those, not find new uses for them.