Yeah that is a very deep wire bite. If you waited any longer to remove it, the wire might have become a permanent feature.

Looks like you removed the wire just in time for the heart shaped cherry tree.

Choosing a fertilizer is a learning process for me. Ultimately, I don't think the wrong fertilizer will bother the tree too much, as long as it is not administered too heavy or weak for the tree. Some trees want a different ratio of NPK during different times of year, to help either the roots, the flower buds, or the foliage green up. You don't want the tree to always focus on greening, all year, or else it will form leaves too big and long internodes between leaves. And you don't want it building oversized roots all year, or else you will have to repot it too often to remove circular roots. Learn the balance that works best for most of your trees. Some trees you may discover have special needs, such as roots preferring aged decomposing bark to maintain temperature, moisture, and even supply of nutrients, and not tolerating clay or sandy soil.

The Rose Society in my area is reputed by bonsai enthusiasts to have the best quality slow-release organic fertilizer for bonsai in my area. They make their own hand-made fertilizer cakes, which they call "poo poo balls", but I have never been able to get my hands on it. Instead I use commercial fertilizers, which are slow-release which I sprinkle onto the soil surface.

One I found a Fruit and Nut tree fertilizer, and it was pretty effective. Last year I use oszmocote, which are little white balls that dissolve slowly, and I do not like those as much. Diluted compost tea is always good when I have enough for weekly Summer feeding. Every year I also sprinkle in a tablespoon or so of a vitamin/mineral rich soil additive, to include trace elements trees need. The one I use is called Azomite. A little bit of Epsom salt a few times a year will not harm trees, and it will soften the water and allow some trees to better absorb those trace nutrients.