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I’d like to know what kind of organic fertlizer is recommended to thicken the trunk of my bonsai. Any specific products mentioned will be appreciated.

There is one, the best one, foolproof.
It's called "time", aka years or decades.

Right fertilisers are not used to nor will they cause a trunk to thicken. Time and in the ground.

The same fertilizer that Will grow most leaves Will grow the thickest trunk. Most of the trunk is really just dead wood, Only cambrium layer is alive.

Plant the tree in the ground Will make more for leaf production and trunk thickness than any fertilizer.

Get a fertilizer with a high N-level. (The label will have N-P-K and then three numbers. The first if for nitrogen, N, which is the predominant driver of explosive growth.