Capsizing a dinghy is not something to be feared, it is something that should be accepted as inevitable, and prepared for accordingly. Dinghy sailing IS a water sport, and you should expect to get wet.
Everything you carry in the boat should be secured to the boat so that you don't have to contend with a debris field when you dump. Centerboard, mast and rudder especially! I am sure you have learned that lesson!
You said swimming is impossible in a lifejacket? Don't use a lifejacket. They are intended to keep you afloat face up until someone can rescue you. As a dinghy sailor you need to "rescue" yourself! Get a decent PFD that will allow you to function in the water as well as having freedom of movement while sailing. (Plus you won't look like a complete noob on the water!
Consider installing a self bailer in your cockpit. Then all you need to do is get the boat moving and the water removes itself.
Certainly you don't want to turtle in shallow waters; High performance dinghies will often break their masts when they do, but unless you are always sailing in shallows you shouldn't need to put a float on your masthead. They do it on cats because they can be very difficult to right due to their beam. Even so you usually only see them on rental boats. They are kinda like training wheels on a bike!
It's better if you get out and practice righting your boat.
When you do dump it, don't try and hang on as long as you can. if you hang on to a capsized boat your body weight is just going to force it to turtle faster. Either jump in the water, or learn to do what the experts do...When the boat is going over, you go over the high side and slide down the hull to stand on the centreboard. Sometimes you can even climb back into the boat as it comes back upright without even going in the drink!
Even world class dinghy sailors capsize, the only difference is how quickly you can recover and be off sailing again!