Welcome to the boards. I started on that tube and everything you are mentioning is spot on about the tube.
1) The caddis has an inflatable backrest that is cylinder shaped and bulges too far forward making your back curve with the backrest which is super uncomfortable. Then the "seat" is just a sling that gets pretty damn slippery when wet. Every time I would get sore and try to readjust my body I would almost slide right out of the seat. Tubes like that fish cat, odc420, and the Cumberland have foam seats and adjustable foam back rest that let you adjust how far you lean back. Some tubes even have inflatable seats. I forget which one it is though.
2) The spinning can be caused by either current or wind. You're not kicking but you're still moving due to current/wind and the tubes are designed to move a certain why. If you are sideways it will straighten you out to match. You have to constantly keep kicking in order to maintain position. How hard you have to kick depends on the current/wind.
3) Yes fish cats cut the water better than your caddis. When I had my caddis I would be in the water up to my belly button. With my Cumberland the waterline is right below my kneecap. The fish cat sits about the same height as the Cumberland.
4) No disrespect intended but the difficultness of removing the crossbar has to do with your weight. When you sit in the tube it kind of wants to close like a taco which adds pressure to the crossbar which is trying to prevent the tube from folding like a taco. The more weight in the tube, the more pressure on the crossbar. If you are thinking about upgrading I would recommend with sticking with one of the three tubes I named earlier and pay attention to the weight capacities. For example, the Cumberland has a weight rating of 350. This gives plenty for you, all your gear, and you even have some to spare. For the fins I find it easier to put them on and take them off while sitting on the tube. That way you don't have to worry about tripping with the fins on. If you can't put the fins on while on the tube then the next best thing is to walk backwards with the fins on. This makes it way easier than walking forward.
Hopefully you do a lot better on your next outing!
If you post your location or the general area that you like to fish there might be someone in the same area that can show you the ropes.