Sous Vide Frustrations
For your steaks, 5 minutes of searing seems like a pretty long time, easily enough to go from medium-rare to medium, or even more than that, depending on thickness and exactly how hot the pan is. It's definitely normal for the meat to be gray before the sear, so if your pan was indeed as hot as you can get it and you needed the whole 5 minutes to get the level of sear you want, you may want to deliberately undershoot more with the sous vide. You may also want to pat the steaks dry, if the liquid from the bag is possible taking you too long to cook off.

For the burgers, you don't mention ever salting them, which would certainly make them bland. You shouldn't mix salt into the patties, but salting just before bagging should be fine, or at least before searing. J. Kenji López-Alt suggests salting before bagging and before searing. On top of that, sirloin steak is pretty lean, which would account for the dryness and maybe some blandness. You might want to try a fattier cut, or part sirloin and part something fatty. People have all kinds of opinions about what's best, but I don't think many people like it as lean as pure sirloin.

In general, salting before bagging is totally fine and often recommended - yes, it might pull a little more liquid out, but if you want salt for flavor, it's going to be a lot more effective in the bag than added at the end. And you're going to get a lot of liquid in the bag no matter what you do. It doesn't mean the meat won't be good, although you can make a great pan sauce out of it and get even more flavor.

Poor circulation, including bags getting stuck against one side without water getting around the other side, definitely isn't ideal. It's hard to tell how big an issue it is for you, though. The best way to check would be just to sneak a thermometer probe to the places you think may not be getting good circulation and see how big a difference there is.

The pot size depends a bit on what you're making. A big stock pot is definitely big enough for a lot of things. I think the one I use is 10-12 quarts, and I do big stuff like pork shoulders. If all you're doing is a couple steaks or fish filets, you can get away with smaller.