What's the best/healthiest thing to do for my 17+yr old 3 feet chestnut tree in a pot so it will survive? I wanted to grow my own bonsai plant from a chestnut then didn't have the nerve to prune it. Is it suitable for a bonsai? It loses its leaves early in the summer most years. Is direct sun best? Is it too late to prune/make into a bonsai? How can I feed it? Should I set it 'free', if so where's best? I just have a south facing balcony or a semi shaded eastern area for plants.
Your Tree already is bonsai! Recommendation is to do fairly much Exactly what you've been doing!! And please do post what the care has been & the soil!! Chestnuts make excellent bonsai, as do many hardwoods & fruit trees, but aren't seen as often!! And, Your Tree sounds very 'happy', and would leave it how it is!
Maybe at this point it could use replacement of the soil, or perhaps a slightly larger container. But do so very carefully and make sure it remains at exactly the same amount of soil leading up to the trunk! And keep it oriented in the same direction. Would use exactly the same soil and blend if at all possible, and exactly whatever nutrient you've been using, and please do post that information here! many of the micronutrients in the soil have been depleted by now, so attempting to change some of the soil, with exactly the same soil as is presently there, would be good. (addendum: apparently you have been doing exactly that, and maintaining the nutrients & micronutrients perfectly!)
As far as pruning goes, the placement of the Tree in the container has reduced its growth, which is exactly what bonsai are. And some bonsai are allowed to grow naturally: that is part of the Art of Bonsai: You know your Tree, and that's how it is perfect! A small amount of judicious pruning might shape it or retard growth, but it seems manageable in size: some bonsai are much larger than Yours.
Might want to possibly spread the branches a bit, depending on how they currently are, in order to adjust the shape to that of a more mature tree, unless it already is, which can be done patiently with padded wires, cables/strings, braces, and weights.
Also, the container dimensions and trunk diameter are important. If want to train it into a gnarled shape, can do that with padded forming wire and weights over a period of years. Or even decades. But it mightn't do as well.
If decide to replant it, even in the same container, would suggest Against much root pruning, and suggest Against trying to remove all the soil. Even though its very nicely established, the tiny roots are still very fragile, and even the weight of the Tree itself can cause harm to them with fresh unfirm soil. Sometimes padded wires are used in the soil to help support a Tree when soil is changed. But that begins to become very specialised, and the best results that could be hoped for, you already have achieved! Excellent!
Hardwood varieties develop very stiff trunks and branches, so if shaping, need to bear that in mind, since tension can cause splitting and separation which might even be difficult to see. So all shaping of established branches & the trunk must be done very patiently & very gently. But yes, that can be done to a surprising extent, it merely takes a while! And its sometimes even possible to move banches to new locations!