Help for thin flavorless jam

If it's getting cooked too much, well, stop cooking it. You can cover it to prevent water loss. If it's too hot sitting on the same burner (on an electric stove), move the pot. It sounds like your cast-iron pot might be part of the problem, too - if it's still staying too hot, you could pour it into something else. Finally, if you do still have significant water loss, I believe you should be able to add a little water (careful not to add too much) and stir and reheat.

P.S. you could also just get a lighter pot. (I know that's not ideal.)

If your jam has at least 1:1 ratio (1 kg of sugar per kg of fruit) or more, you do not have to can it. Then it is so overwhelmingly sweet that bacteria cannot live in it.

If the jam has less sugar (1:2 are popular, 1:3 are found sometimes), then you have to either can it, or keep it in the fridge and consume it within a few days, similar to any other cooked food. Else, you are colonizing bacteria in your jam, and at some point, some of them will happen to be pathogenic and cause some unpleasant disease in whoever eats the jam.

Canning is a process in which the jam is closed against any new bacteria (that's why you have to close the jar hermetically) and then heated enough that most old bacteria are killed. After canning, you can keep the closed jars at room temperature without any new bacteria developing.