Paddling in one side in a straight line or corrections in direction are quite simple and easy if you have a reasonable technique. I tend to only change sides for a change if I’ve been paddling on one side for a while or if there is a something like a cross wind, current or swell that it’s easier to paddle against from one side.
The fin at the back isn’t a rudder it’s job is to keep the board going straight ahead in the direction of travel. If you find a board unstable you can use a larger “touring” fin as it will resist being pushed sideways through the water more when you wobble.
I think stern rudder your talking about using the paddle as a rudder at the back. Therefore, fighting against the fin, creating lots more drag and slowing you down like putting the brakes in. If you use the paddle as a rudder at the front it will have a much greater effect on turning the board, start of carefully and brace your self because it can have a lot of leverage on the paddle and not be in a particularly strong stance.
The biggest effect on direction are from the paddle furthest away from the fin as the board will mostly rotate about it (crossbow turn).
The angle and direction of the paddle on the first part of the paddle stroke in the water will have the largest effect on turning the board or keeping it going straight. So a canon style J stroke is less efficient.