You can see from the photo below the state of a leaf that doesn't seem too healthy. Is there something more that I should be doing? Could the brown spots be leaf scorch? I'd like to see this plant succeed over the coming years and hopefully develop into something worth training into a bonsai.
In this season, some damage to the leaves is normal. I love spring - when all the leavers are fresh and undamanged. But I too see damage on my trees from bugs eating the leaves. It's normal and not alarming.
So far I think you're doing fine, but do realise it is going to take you years to develop a decent trunk before this will become suitable for bonsai.